My client's site has been almost entirely deindexed

Hey, I'm looking for other brains to help me understand what happened to my client's site as I'm struggling to work this out; all help is appreciated.

Site in question: https://get.fitr.training/

Here's the story of my investigation so far:

  • On Around Wed 4th Sep, I started to see some pages being deindexed (coincides with the algo update finishing)
  • Slowly since then, all but 10 pages (according to Search Console) are now deindexed. However, I suspect once those pages are recrawled they'll also be taken down.
  • I have checked multiple times, no DMCA takedown has been issued.
  • There was a sitemap error showing for some crawled pages, but not for others. Across all deindexed pages, I saw 'no referring sitemap', 'temporary processing error', and the link to the correct XML sitemap. The sitemap was previously autogenerated using Webflow. I have created a manual sitemap to try and help, but I don't believe this is the issue. A
  • There was no robots.txt file. I have since added one just to tick it off the list, but again this shouldnt cause this deindexing issue.
  • I have been fixing minor technicals, such as links to redirects, etc. Again, in my opinion, none of these would cause this and I'm mostly doing it to tick them off the list.
  • All of the website content is unique and is written to explain what their software is and the benefits it has to their audience.
  • We have never purchased backlinks, so no worries of spammy backlinks. I've also checked Ahrefs and nothing here is causing me concern.
  • Core web vitals are fine.
  • Nothing in 'Removals'.
  • Sitemap is valid in Search Console.
  • No other errors I can really see.

I have a weak theory that this has something to do with the name 'FITR'. Fitr is also the name of a religious festival, and I'm wondering if the algo update deemed their site as completely unrelated and spammy because of it. People do search for their brand name, but the majority of that search volume will be for the festival. I am however stuck because I don't think that would cause them to deindex pretty much the whole website.

I have reached out to the Google community who think it's strange, and I've reported an indexing bug to Google. This was 10 days ago and still nothing back, so here am I hoping someone here might have some ideas, or at the very least agree that this is a strange one.