Cannot resolve domain

This is completely out of context. Happy eyeballs is for clients PCs, not for DNS recursors. The specific RFC section you mention is for clients accessing their own resolvers, not for resolvers accessing authoritative DNS servers, and it is there because clients may or may not have intermittent IPv6 connectivity.

However, an authoritative servers must be correctly configured. If you provider doesn’t, and doesn’t fix this issue immediately when reported, then that provider is obviously completely incompetent. won’t be your last problem then, you will keep hitting this problems with various other providers.

The same exact issue happens with SMTP/MX records: there as well you have the same problem. If service providers are not competent enough to respond in IPv6, they have to drop the AAAA entries from their records. Just because Cloudflare does deploy a fix or not, and just because “it works with Google” doesn’t change the problem that you will continually face issues due to this missconfiguration on your service providers side.