A `$derived` property depending on a `SvelteSet` doesn't update

[abdel-17]

Describe the bug

In this example, I have an expanded_ids set. Each item has an expanded property that depends on that set. For some reason, the expanded property doesn't update when you read it immediately after updating the set. I found a strange workaround for this bug. You can find it in the playground below.

Reproduction

https://svelte.dev/playground/6f5ca2c4404d4c57a953869b58b954cb?version=5.45.2

Logs

System Info

System:
    OS: macOS 26.1
    CPU: (10) arm64 Apple M4
    Memory: 184.72 MB / 16.00 GB
    Shell: 4.2.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 22.21.1 - /Users/abdel/.local/share/mise/installs/node/22.21.1/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 10.9.4 - /Users/abdel/.local/share/mise/installs/node/22.21.1/bin/npm
    pnpm: 10.20.0 - /Users/abdel/.local/share/mise/installs/pnpm/10.20.0/pnpm
    bun: 1.3.3 - /opt/homebrew/bin/bun
  Browsers:
    Brave Browser: 142.1.84.141
    Safari: 26.1
  npmPackages:
    svelte: ^5.45.2 => 5.45.2

Severity

annoyance

[GrygrFlzr]

The odd behavior appears to begin with version 5.43.4, so most likely the PR #17105 is relevant.
I also prepended <svelte:options runes /> to the App.svelte just in case it was a behavior oddity between Legacy mode and Runes mode interacting, but forcing runes mode on both still produce the behavior.