`{@const}` inside `{#each}` goes stale when prop-derived value is only consumed inside an array callback

Describe the bug

A {@const} inside {#each} does not invalidate when its reactive dependency arrives via a prop and that dependency is only referenced inside an array callback (.find(), .filter(), .some(), etc.) rather than in the outer expression.

From the template's perspective the prop changed, a sibling {@const} that reads the prop directly updates correctly, but the {@const} whose expression only touches the prop inside a callback closure stays frozen at its previous value.

Required conditions

Both must be present:

1. The reactive dependency is a prop — does not reproduce when the same value is a local variable or a store.
2. That prop value appears only inside the callback of the array method — removing it from inside and placing it in the outer expression fixes the staleness.

No transition, no {#if} toggle, no async wrapper needed.

Reproduction

Link to REPL https://svelte.dev/playground/9e909f44f22a406a80a639c6ecd69496?version=latest

The playground shows both the buggy and the fixed pattern side by side, driven by the same prop. Click any button — selectedItemId updates correctly in both sections, but only the Fixed section reflects the change in the derived selectedItem.

Expected behavior

Clicking any button updates selectedItemId. Both sections should then resolve selectedItem and show the correct seat count.

Actual behavior

selectedItemId updates in both sections. The Bug section never shows seats — selectedItem stays undefined on every click. The Fixed section works correctly.

Workaround

Make the prop-derived scalar appear in the outer expression before the callback:

<!-- Buggy: selectedItemId hidden inside callback -->
{@const selectedItem = group.items.find((item) => item.id === selectedItemId)}

<!-- Fixed: selectedItemId visible in outer expression -->
{@const selectedItem = selectedItemId && group.items.find((item) => item.id === selectedItemI