diff --git a/core/activity.py b/core/activity.py
index e611febd..6dfec704 100644
--- a/core/activity.py
+++ b/core/activity.py
@@ -124,8 +124,18 @@ class FileActivity:
     run_id: Optional[str] = None
     run_source: str = "legacy"  # "scheduled" | "web" | "cli" | "maintenance" | "legacy"
 
-    def to_dict(self) -> dict:
-        fmt = get_time_format()
+    def to_dict(self, time_format: Optional[str] = None) -> dict:
+        """Serialize for the dashboard/API.
+
+        Args:
+            time_format: "12h" or "24h". When None, read once from settings.
+                When serializing a LIST, the caller should read the format once
+                and pass it to every entry. Calling get_time_format() per entry
+                re-reads the settings file each time, so a concurrent settings
+                write can make one entry fall back to the "24h" default and drop
+                its AM/PM suffix while its siblings keep theirs.
+        """
+        fmt = time_format or get_time_format()
         if fmt == "12h":
             time_display = self.timestamp.strftime("%-I:%M:%S %p")
         else:
diff --git a/core/file_operations.py b/core/file_operations.py
index 8cc3a633..ecd2c59f 100644
--- a/core/file_operations.py
+++ b/core/file_operations.py
@@ -49,16 +49,20 @@
 ARTICLE_SUFFIX_RE = re.compile(r'^(.*?),\s+(The|A|An)(\s*\(\d{4}\))?\s*$', re.IGNORECASE)
 
 
-def save_json_atomically(filepath: str, data, label: str = "data") -> None:
+def save_json_atomically(filepath: str, data, label: str = "data") -> bool:
     """Save JSON data to file atomically (write-to-temp-then-rename).
 
     Creates a temp file in the same directory, writes data, then atomically
-    replaces the target file. This prevents corruption from interrupted writes.
+    replaces the target file. This prevents corruption from interrupted writes
+    and ensures concurrent readers never observe a truncated/partial file.
 
     Args:
         filepath: Target file path.
         data: JSON-serializable data to write.
         label: Human-readable label for error messages.
+
+    Returns:
+        True if the file was written and replaced successfully, False otherwise.
     """
     try:
         dir_name = os.path.dirname(filepath) or '.'
@@ -73,8 +77,10 @@ def save_json_atomically(filepath: str, data, label: str = "data") -> None:
             except OSError:
                 pass
             raise
+        return True
     except IOError as e:
         logging.error(f"Could not save {label} file: {type(e).__name__}: {e}")
+        return False
 
 
 def is_subtitle_file(filepath: str) -> bool:
diff --git a/tests/test_activity_feed.py b/tests/test_activity_feed.py
index 45a3c24b..23b5da19 100644
--- a/tests/test_activity_feed.py
+++ b/tests/test_activity_feed.py
@@ -471,3 +471,33 @@ def test_summary_written_atomically(self, tmp_path):
         keyword = call_args[1]
         label = keyword.get("label") if "label" in keyword else (positional[2] if len(positional) > 2 else None)
         assert label == "run summaries", f"Expected label 'run summaries', got {label!r}"
+
+
+# ============================================================================
+# save_json_atomically success/failure reporting
+# ============================================================================
+
+class TestSaveJsonAtomicallyReturn:
+    """save_json_atomically() must report success so callers (e.g. the settings
+    writer) can preserve their bool contract."""
+
+    def test_returns_true_on_success(self, tmp_path):
+        from core.file_operations import save_json_atomically
+
+        target = tmp_path / "out.json"
+        ok = save_json_atomically(str(target), {"a": 1}, label="test")
+
+        assert ok is True
+        assert json.loads(target.read_text()) == {"a": 1}
+
+    def test_returns_false_on_write_failure(self, tmp_path):
+        from core.file_operations import save_json_atomically
+
+        target = tmp_path / "out.json"
+        # Simulate a disk/IO failure during the atomic replace.
+        with patch('core.file_operations.os.replace', side_effect=IOError("disk full")):
+            ok = save_json_atomically(str(target), {"a": 1}, label="test")
+
+        assert ok is False
+        # Original target must not be left in a partial state (temp is cleaned up).
+        assert not target.exists()
diff --git a/tests/test_core_activity.py b/tests/test_core_activity.py
index e52a3bd2..21f5ae06 100644
--- a/tests/test_core_activity.py
+++ b/tests/test_core_activity.py
@@ -70,6 +70,37 @@ def test_to_dict_12h_format(self):
 
         assert "PM" in d['time_display']
 
+    def test_to_dict_explicit_time_format_skips_settings_read(self):
+        """Passing time_format must override settings and not read them.
+
+        Guards the render-consistency fix: when serializing a list, the caller
+        reads the format once and passes it, so a concurrent settings write
+        can't make one entry fall back to the 24h default mid-render.
+        """
+        fa = FileActivity(
+            timestamp=datetime(2026, 1, 15, 14, 30, 5),
+            action="Cached",
+            filename="movie.mkv",
+        )
+        with patch('core.activity.get_time_format') as mock_get_fmt:
+            d = fa.to_dict(time_format='24h')
+
+        mock_get_fmt.assert_not_called()
+        assert d['time_display'] == "14:30:05"
+
+    @pytest.mark.skipif(sys.platform == 'win32', reason="%-I strftime is Linux-only")
+    def test_to_dict_explicit_12h_overrides_24h_settings(self):
+        """An explicit 12h arg wins even when settings say 24h."""
+        fa = FileActivity(
+            timestamp=datetime(2026, 1, 15, 14, 30, 5),
+            action="Cached",
+            filename="movie.mkv",
+        )
+        with patch('core.activity.get_time_format', return_value='24h'):
+            d = fa.to_dict(time_format='12h')
+
+        assert "PM" in d['time_display']
+
     def test_zero_size_dash_display(self):
         with patch('core.activity.get_time_format', return_value='24h'):
             fa = FileActivity(
diff --git a/web/services/operation_runner.py b/web/services/operation_runner.py
index 6cace8ba..217426a3 100644
--- a/web/services/operation_runner.py
+++ b/web/services/operation_runner.py
@@ -689,7 +689,12 @@ def recent_activity(self) -> List[dict]:
         Reloads from disk to include entries written by MaintenanceRunner.
         """
         activities = load_activity()
-        return [a.to_dict() for a in activities]
+        # Read the time format ONCE for the whole render. Calling get_time_format()
+        # inside each to_dict() re-reads the settings file per entry, so a concurrent
+        # settings write could make one row fall back to the 24h default and drop its
+        # AM/PM while its siblings keep theirs.
+        time_format = get_time_format()
+        return [a.to_dict(time_format=time_format) for a in activities]
 
     def _parse_size(self, size_str: str) -> int:
         """Parse a size string like '1.5 GB' into bytes."""
diff --git a/web/services/settings_service.py b/web/services/settings_service.py
index dbe9c655..1f1107bc 100644
--- a/web/services/settings_service.py
+++ b/web/services/settings_service.py
@@ -97,10 +97,17 @@ def _load_raw(self) -> Dict[str, Any]:
             return {}
 
     def _save_raw(self, settings: Dict[str, Any]) -> bool:
-        """Save raw settings to file"""
+        """Save raw settings to file atomically (write-temp-then-replace).
+
+        An in-place truncating write (open 'w') lets concurrent readers — e.g.
+        get_time_format() during a dashboard render — observe an empty/partial
+        file and silently fall back to defaults. The atomic replace prevents
+        that: readers always see either the old or the new complete file.
+        """
+        from core.file_operations import save_json_atomically
         try:
-            with open(self.settings_file, 'w', encoding='utf-8') as f:
-                json.dump(settings, f, indent=2)
+            if not save_json_atomically(str(self.settings_file), settings, label="settings"):
+                return False
             # Restrict permissions — settings contain secrets (Plex token, password hashes)
             try:
                 os.chmod(self.settings_file, 0o600)
@@ -108,7 +115,7 @@ def _save_raw(self, settings: Dict[str, Any]) -> bool:
                 pass  # Non-fatal (Windows, Docker with different uid)
             self._cached_settings = None  # Invalidate cache
             return True
-        except IOError:
+        except (IOError, OSError):
             return False
 
     def _sanitize_path(self, path: Optional[str]) -> Optional[str]: