I’ve got the same issue, but I don’t think it’s related to the issue noted above. I enabled HTTP/3 on all my zones about 2 weeks ago, and it didn’t work then (via Firefox which I’ve specifically enabled HTTP/3 on, as well as https://gf.dev/http3-test and https://http3check.net/). All my zones are still showing HTTP/3 as being toggled on in the Cloudflare UI.
I’ve kept trying now and again between enabling it around 2 weeks ago and today, it’s never worked. I’ve come back to this day to see if there’s any obvious misconfigurations, but it looks correctly configured and should be working as far as I can tell.