I cannot see what is causing the issue, the certificate's full chain is sent, the clock is synced, the cert is showing zero errors in OpenSSL - so this is very confusing.

The irony is, you don't actually need Tor on my site because there is no logging, no third parties, no adtech etc. it is just static HTML files - so whereas I would normally recommend Tor I designed the site specifically to be privacy first.

I will try to figure out what is going on though because obviously I am fully supportive of people protecting their privacy with Tor.