`async` experimental flag causes layout navigation failure

[awei82]

Describe the bug

When the async experimental compiler flag is set, client-side navigation fails in one very specific condition:

• A +layout.svelte is present in a route (example: /route1)
• User navigates from that route to another (example: /route2)
• Any follow-on navigation fails to load (navigation from /route2 -> /route3 will fail)

See README in https://github.com/awei82/svelte-async-bug-poc for full explanation of the bug

Reproduction

https://github.com/awei82/svelte-async-bug-poc

Logs

System Info

System:
    OS: macOS 15.6.1
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 219.29 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.2.0 - /Users/andrewwei/.nvm/versions/node/v24.2.0/bin/node
    Yarn: 1.22.17 - /Users/andrewwei/.yarn/bin/yarn
    npm: 11.6.0 - /Users/andrewwei/.nvm/versions/node/v24.2.0/bin/npm
    pnpm: 10.18.3 - /Users/andrewwei/.nvm/versions/node/v24.2.0/bin/pnpm
  Browsers:
    Chrome: 142.0.7444.176
    Firefox: 145.0.1
    Safari: 18.6
  npmPackages:
    svelte: ^5.45.4 => 5.45.4

Severity

blocking an upgrade

[OwLu0905]

Have the same issue. What I found is when navigating like: A → A/[slug] → B → B/[slug], where both A/[slug] and B/[slug] have async call functions. Removing data-sveltekit-preload-data="hover" only works when navigating within the same route group (A → A/[slug] or B → B/[slug]), but the navigation still breaks when switching between different dynamic route groups.

[awei82]

I'd also like to add that this issue only seems to occur in development. It looks like it goes away in the production build ¯\(ツ)/¯

[awei82]

this is likely the same issue as #17209