cached_html_pages does not purge

Huh! that is wired. Can you try the following:
First diable fallback cache and then inside the plugin settings enable the logs and under the Others tab set the log verbosity to HIGH.

Then inside the plugin settings purge the whole cache (I mean just the HTML pages) Screenshot: https://i.imgur.com/UGPCruJ.png .

Then wait for a few minutes and check the logs generated by the plugin to see if all the pages have been purged. Then check the folder to see if those temp files are still there.

It’s best to run this test on a low traffic site so that while you care purging cache others don’t visit the page creating temp files for those pages.

I have not tested as per your instructions yet, but I did notice earlier that if I manually purge (clicking Purge Cache button in plugin or Purge Whole Cache in the admin bar), the tmp files are purged.
However, it seems that the tmp files do NOT automatically purge when the cache expires, but the html files in fallback_cache are purged as expected.

However, it seems that the tmp files do NOT automatically purge when the cache expires

– yes that is right. If you read this article: https://acnam.com/untold-truths-of-cloudflare-cdn/#cloudflare-cache-creation-replication-storage to understand how Cloudflare cache works, there is no way for anyone to know when Cloudflare has deleted the page from it’s cdn cache.

OK. Does that the cached_html_pages directory will continue to grow with .tmp files until cache is manually purged?

yes. that directory is purged when you either to a full manual purge or you update an article and that leads to purging the respective page and it’s related pages.

OK thank you!