Not prerendered getStaticProps html response for fallback: true is not cacheable in next@canary

[pieh]

Link to the code that reproduces this issue

https://github.com/pieh/next-fallback-true-no-cache

To Reproduce

1. Clone https://github.com/pieh/next-fallback-true-no-cache repository.

2. Install deps, build and start the server:

   npm install
   npm run build

3. Start the server:

   npm start

4. Hit not-prerendered page (using example URL, but any will work other than "/prerendered" as there is cacheable catch-all page)

   curl --head http://localhost:3000/not-prerendered

   Notice the response headers (expected, this page was not prerendered and fallback was served, so should not be cached):

   x-nextjs-cache: MISS
   Cache-Control: private, no-cache, no-store, max-age=0, must-revalidate
   

   Now let's hit json endpoint to generate the page

   curl --head http://localhost:3000/_next/data/build-id/not-prerendered.json

   Notice the response headers: (expected, page was not rendered and cached yet, so we get cache miss, but we do get cacheable cache-control header as expected with revalidate:3600)

   x-nextjs-cache: MISS
   Cache-Control: s-maxage=3600, stale-while-revalidate=31532400
   

   At this point next html response should not be fallback anymore, should be cache hit and should have cacheable cache-control header, so let's hit the page again:

   curl --head http://localhost:3000/not-prerendered

   Notice the response headers: (NOT expected, page was served from the cache, but cache-control won't allow to cache it):

   x-nextjs-cache: HIT
   Cache-Control: private, no-cache, no-store, max-age=0, must-revalidate
   

Current vs. Expected behavior

Expected Cache-control header when hitting HTML route after page was generated and fallback was not served anymore should be s-maxage=3600, stale-while-revalidate=31532400 as it was for the json endpoint, but instead got not cacheable private, no-cache, no-store, max-age=0, must-revalidate

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:25 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6020
  Available memory (MB): 32768
  Available CPU cores: 12
Binaries:
  Node: 22.13.1
  npm: 10.9.2
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 15.4.0-canary.94 // Latest available version is detected (15.4.0-canary.94).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: N/A
Next.js Config:
  output: N/A

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

Pages Router, Dynamic Routes

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

next start (local)

Additional context

Last version this worked as expected was next@15.4.0-canary.77. 15.4.0-canary.78 canary is first one that has this regression.

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