`let $derived` object not updating with `$bindable`

[agrattan0820]

Describe the bug

Not sure if this is the interaction that's supposed to happen and I'm meant to use the $state in this scenario instead, but I found that when trying to mutate an object defined with let $derived gets passed down to a bound component it doesn't trigger a reactive update to the UI.

As soon as I change it to a $state rune the UI updates so it's simply an annoyance right now keeping us from removing the usage of some $effect runes.

Reproduction

Svelte Playground: https://svelte.dev/playground/114b3ffd135248c481adce41c3366e26?version=5.36.7

Logs

System Info

System:
    OS: macOS 15.5
    CPU: (11) arm64 Apple M3 Pro
    Memory: 251.31 MB / 18.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.nvm/versions/node/v22.14.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v22.14.0/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.14.0/bin/npm
    pnpm: 10.12.4 - ~/Library/pnpm/pnpm
    bun: 1.2.18 - ~/.bun/bin/bun
  Browsers:
    Brave Browser: 138.1.80.120
    Safari: 18.5
  npmPackages:
    svelte: ^5.36.6 => 5.36.6

Severity

annoyance

[agrattan0820]

Ah nevermind, I assume it is because of this: https://svelte.dev/docs/svelte/$derived#Deriveds-and-reactivity