Svelte 5: broken reactivity in legacy mode in rare case

[7nik]

Describe the bug

A child component may not see updates from the parent.

From Ennoriel/svelte-awesome-color-picker#69 task 1.

Reproduction

REPL 5
REPL 4

Logs

No response

System Info

Svelte 5.0.0-next.134

Severity

blocking an upgrade

[rChaoz]

Slightly more simplified REPL.

It seems like $.derived(() => x) is not getting updated when $.mutate(x, $.get(x).y += 1) occurs.