`async` breaks reactivity with SvelteKit's preload

[raphael-zenhaeusern-legic]

Describe the bug

Might be somewhat related to #17209. While the issue shows in kit, I assume the cause is somewhere within svelte proper, so filing here.

When using async in SvelteKit, preloading data may cause a loss of reactivity. While it seems that the value is updated (at least $inspect() suggests as much), the $effect() is not executed.

This can be triggered by both using data-sveltekit-preload-data or making use of preloadData().

Reproduction

https://stackblitz.com/edit/sveltekit-reactivity-loss?file=src%2Froutes%2F%2Bpage.svelte

Initially, opening the dialog both via button or the anchor works.
Once root +page.svelte is adjusted by either setting triggerError to true or data-sveltekit-preload-data to hover, opening the dialog via the button only works as long as the link has not been hovered or clicked.

Inspecting dialogShowing in lib/SvelteDialog.svelte indicates the variable is updating but the effect not firing.

Logs

System Info

System:
    OS: macOS 26.1
    CPU: (10) arm64 Apple M2 Pro
    Memory: 65.02 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.11.1 - /opt/homebrew/opt/node@24/bin/node
    npm: 11.6.2 - /opt/homebrew/opt/node@24/bin/npm
  Browsers:
    Edge: 142.0.3595.94
    Firefox: 145.0.2
    Safari: 26.1
  npmPackages:
    svelte: ^5.42.2 => 5.45.5

Severity

blocking an upgrade