`fork` breaks reactivity of class fields in object from a `$derived`

[zqianem]

Describe the bug

After the fork is committed, updates to a class field where the object is returned from a $derived (or a $derived.by) are no longer reflected in the DOM.

See comments in the reproduction for details.

(Found in a SvelteKit project, where the original $derived.by actually has dependencies.)

Reproduction

https://svelte.dev/playground/hello-world?version=5.43.3#H4sIAAAAAAAAA41S227CMAz9FStDohWo3TOUStNe9gto7KG07ohIk6oxFFTl3xcHEAVt0h597OPLOR6ELhoUC_GBShnoTacqiLCShFUs5qKWCq1YfA6Czi3XMeDxK-utbRN7REWMbQuLv-Gl0YSafBuR2bKTLeUbvSHZtKYjGKA23R4c1J1pYHphTZejived9Etd0kkaomRclqagkOAEK5hYKggj6g4YLwF8ppe0MwcKQ6T-nkOpZLkHqcsOG96KOzzR60LZMf-ZXBm0oA3tPLbRWXo_SmfbA5HRYHQoXQ1RDKscBp7idbB-ET-H-0WjzIZ4Om_NkYvDWXVSmqaRFHHoXM6kLL30D7OGF1nDyXFtdtEo9fiQytp51QlPJBbc083_cG8s5aN_T5k_HCxVYS2sbzfUxtw1fI3DKSN_zj6psYd1dJXW62f54_b2QeWbIVw_qbCTR6yiK3HMZBf0lP5nwDnx281mLg_oSMaszYeQdFna5o-6ffmokKqXuhKL8BTuB2XYA4cwAwAA

Logs

System Info

n/a

Severity

blocking an upgrade

[zqianem]

Stranger still — adding an await before the $state declarations makes it work again:

https://svelte.dev/playground/hello-world?version=5.43.4#H4sIAAAAAAAAA41SwW7bMAz9FUIrEBsN7J1Tx0PRy467FssOqi03QmTJkNg6gaF_HyknqBOs6A4-iHzv-ZGPk7CyV2IjfipjHIzOmxYy1WpUbS7WotNGBbH5PQk8DYzjAtXPrMdhKMK7Msi1FxnUv-qNs6gskoyoQuP1gPXO7lD3g_MIE3TOHyBC510Pq5m1elggnvaaTM3tokyvYgnjT45SI1g1wi_C6aAyD9safJbnD1CWINtW21fAvQ7Qy4MKQHCa9vDjokAgoxCOsIW7gBJVhv5NEZvpo8a9e8NklXTW0BjdHEDbxqueZ2OFG3onTVjyb8mtIxfWkSX7SvTZwldzvJAJ4sBeebWzVfmxT1tRD50FZ5P-dspypk6sTBEEck_m2ES26OyQLfOo_Ip52mhXNK7vNWb8jLFmUlXO-ulf0zfdwTEytprjKak-lbqLFDiqI4oNa8b1J4ezTPH6dG46nxxPY2QI8HyZoXPuY_Hf8zTKItQTNXmnz9k5j3QHHH-4iuaSIuPvWuX1u2qzM3HJ5OjsCv8vgFNB7u7vY52qizVWQz2lZqzKob7e2x96SW1GbVuxSZcU_wJABkzKqwMAAA

Also noticed that the test case here is similar to the one for #17098; could it be related?