It seems that it checks for an A record (ipv4):
I didn’t find the referring code (in the main server repository), so I don’t know if that could be easily adjusted for dual-stack and/or ipv6 only. You can check the availability from outside your host via ipv6, if that is working, you can consider this just as an invalid warning.
In such a case, you need to be careful to share links, since not everybody has ipv6. Even though, in some countries, ipv6 coverage reaches 70-80%, even on mobile networks.