Page is still served from cache if getStaticProps() returns "notfound: True"

[jorups]

Scenario:

• I'm using Headless CMS and SSG + on demand revalidation
• Next.js app is built and all existing pages are rendered and served - all good
• In headless CMS new page with path "/testpage1" is created
• Headless CMS calls next.js app's hook that calls revalidate("/testpage1")
• New page "/testpage1" is rendered and visible - perfect
• Page is deleted from Headless CMS, that calls revalidate hook again
• Page "/testpage1" still is served by next.js app - not expected

I'm using pages and in getStaticProps () I return { notFound: true, revalidate: 5 } when trying to fetch data for the page from Headless CMS that doesn't exist.
For other pages that don't exist, app is returning 404 - that is correct. However, deleting the existing page and calling revalidate() doesn't purge the cache, and the page is still visible.

I'm not 100% sure it is related to cache handler but to the logic on next.js side that decides when to re-render specific page. Given the fact that revalidate() eventually calls cache handler's revalidateTag it should trigger rerendering the page and return 404 if { notFound: true, revalidate: 5 } is returned by getStaticProps(), no? Or am I missing something?

P.S. With NEXT_PRIVATE_DEBUG_CACHE set to true I don't see any output of revalidateTag, so not sure that revalidateTag is triggered during on demand revalidation.