I had a number of pages that Google's Search Console had marked as "Discovered - currently not indexed" 2 weeks ago, and so I clicked "Validate Fix" in the hopes of having the pages properly indexed. Once Google told me the fix had been validated, the pages disappeared off that list, but also weren't indexed.
If I check them on the URL inspector, it now says "URL is unknown to Google". All the pages have links to them from the homepage (which is correctly indexed), and they're mentioned in the sitemap, which says it successfully discovered 21 pages. Despite that, it says "no referring sitemap detected".
I don't know if it's relevant, but the "See page indexing" button on the sitemap is greyed out. Additionally, these pages have been up for about 4 months now, having only had this activity now, 2 weeks ago.