Incorrect caching behavior with revalidate time on unstable_cache

[mastoj]

Link to the code that reproduces this issue

https://github.com/mastoj/next15-caching

To Reproduce

1. Clone repo
2. Deploy to vercel
3. Go to /whatever
4. When any of the timers are up click refresh

Current vs. Expected behavior

When any of the timers are out I would expect to get ONE STALE response and then the next refresh should load a new image for the timer that has ended and the timer should also be reset.

Instead it seems like I am always getting "old" cached data in my result.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:23 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6020
  Available memory (MB): 65536
  Available CPU cores: 12
Binaries:
  Node: 20.14.0
  npm: 10.7.0
  Yarn: 1.22.22
  pnpm: 10.4.1
Relevant Packages:
  next: 15.2.0 // Latest available version is detected (15.2.0).
  eslint-config-next: 15.1.6
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

Vercel (Deployed)

Additional context

I noticed this when working on a demo application where I try to leverage unstable_cache with both revalidate time and revalidation tags. The revalidation tags works fine, but not the revalidate time. When the time is out the STALE response is served and that is also cached on the edge it seems, and when the revalidated data is ready it won't update the edge cache so I am still serving cache data. Then when the second timer is out it will serve the previous data because that is the last one cached.