Why doesn't the TXT entry in my DNS record get propagated across the web?
This is the DNS stuff they don't tell you.
It seems that if you've got a CNAME and a TXT for the same subdomain (y'know, that bit in your Namecheap etc settings called 'Host'), only one of the two will ever get distributed. CNAME takes precedence over TXT, so odds are your TXT entry will just stay in your settings panel forevermore.
I had this problem while migrating to Bear Blog. It required me to have a CNAME entry for the '@' host pointing to 'domain-proxy.bearblog.dev'. Because I use Protonmail to host my email with the same domain, I was getting URGENT NOTICE emails within twenty four hours saying that the entries to direct my email properly were no longer present. Protonmail requires a TXT field for the '@' host too (something like 'protonmail-verification=xxxxxxxxxx'). The CNAME is the important one, the TXT one gets overlooked, hence: problems.
What's the solution?
If you're having this problem, try updating the CNAME to an ALIAS. It does moreorless the same job, works happily with TXT records on the same subdomain, and has made both Bear and Proton happy. For now.