State nested inside of SvelteMap is no longer reactive

[WaltzingPenguin]

Describe the bug

If SvelteMap is used to access an object containing state, the result is no longer reactive sometime after 5.23.0.

Reproduction

https://svelte.dev/playground/8f3e032e86604e9db9c4de89c39707f0?version=5.23.0
Pinned against 5.23.0
Works as intended, it displays "Loading..." and then is replaced with an image of Eevee

https://svelte.dev/playground/8f3e032e86604e9db9c4de89c39707f0?version=5.28.2
Latest Version
View is stuck displaying "Loading...". No errors or warnings are issued. Changing the id to 132 and then back to 133 will cause the view to update correctly.

Logs

System Info

Reproducible with web REPL

Severity

blocking an upgrade

[harrisi]

This seems to be caused by #15564.

EDIT: bisected to 5.24.0 being the breaking change, and tested all other commits between 5.23.3 and 5.24.0.