can't refine Dict[str, str] with isinstance check

[sbdchd]

Bug Report

Refining x: Dict[Any, Any] works via isinstance(x, dict), but doesn't for x: Dict[str, str]

To Reproduce

from typing import Dict, Any

z: Dict[str, str]
if not isinstance(z, dict):
    reveal_type(z) # note: Revealed type is 'builtins.dict[builtins.str, builtins.str]'

x: Dict[Any, Any]
if not isinstance(x, dict):
    reveal_type(x) # error: Statement is unreachable

Expected Behavior

Dict[str, str] would be refined such that mypy gives a statement is unreachable error

Actual Behavior

Dict[str, str] doesn't get refined

Your Environment

• Mypy version used: 0.7.9
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9
• Operating system and version: linux

[AlexWaygood]

Can't reproduce on 0.940+