Domain validation via (correct) _dnsauth.<subdomain> TXT record not working in Front Door Standard/Premium after a couple days

Question

I'm having trouble on the Domains blade of my Front Door Standard/Premium resource, with the domain validation step. "Validation state" is "Pending", and when I click on that link, the "Validate the custom domain" panel gives the information for the DNS record required in order to validate the domain. It's a TXT record for _dnsauth.<subdomain>.<domain>.com with a specific record value. It has a "Refresh status" button and a warning above it: "Please refresh status after above verification challenge is completed. The token validation may take a while to take effect." I created the correct TXT record a day and a half ago and have verified the record is correct using multiple DNS services: Google, Cloudflare, OpenDNS, etc. I've checked that the record name and record value are precisely what Azure is asking, character for character. Despite that, I cannot get it to validate the domain so that I can begin using it. What am I missing?