Bug revalidating ISR routes with optional params on Vercel

Describe the bug

I noticed that ISR routes with optional parameters were not being revalidated properly on Vercel.

After revalidating, refreshing the /route/[[params]] in the browser does show the fresh version. However, the stale version is still served when trying to navigate to it.

This isn't an issue with routes with [param] or no params at all.

Reproduction

https://kit-isr-demo.vercel.app
https://github.com/adrienpicard/kit-isr-demo

Revalidating routes with x-prerender-revalidate seems to only partially works on routes with optional [[params]]

Route A (no optional params) /route-a

1. Load Route A in Tab 1
2. Open the landing page in Tab 2 and click Invalidate
3. Navigate to Route A from Tab 2, the page was invalidated ✅
4. Refresh Tab 1, the page was invalidated ✅

Route B (with optional params) /route-b/[[myparam]]

1. Load Route B in Tab 1
2. Open the landing page in Tab 2 and click Invalidate
3. Navigate to Route B from Tab 2, the page was not invalidated ❌
4. Refresh Tab 1, the page was invalidated ✅

Logs

System Info

System:
    OS: macOS 14.6.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 211.48 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.18.3 - ~/.nvm/versions/node/v20.18.3/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v20.18.3/bin/yarn
    npm: 10.8.2 - ~/.nvm/versions/node/v20.18.3/bin/npm
    pnpm: 9.7.0 - ~/.nvm/versions/node/v20.18.3/bin/pnpm
  Browsers:
    Chrome: 135.0.7049.96
    Edge: 135.0.3179.73
    Safari: 17.6
  npmPackages:
    @sveltejs/adapter-auto: ^4.0.0 => 4.0.0 
    @sveltejs/kit: ^2.16.0 => 2.20.7 
    @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.0.3 
    svelte: ^5.0.0 => 5.28.0 
    vite: ^6.2.5 => 6.3.2

Severity

serious, but I can work around it

Additional Information

No response

Start with the linked kit-isr-demo reproduction and compare navigation versus refresh for /route-a and /route-b/[[myparam]] after invalidation. Trace how x-prerender-revalidate handles optional parameters, and consider the issue done when navigation serves the fresh version as refresh already does.