For domain name github.com (which sometimes returns no A record), I always receive an answer with TTL set to 60. It seems like this name does not get cached at all. See the test output below.

Interestingly, if I delete cache files and restart the resolver, the TTL starts decreasing as expected. Is this a sign that something was wrong with the cache before? Or is this some sort of cache optimization for low TTL records?

Nothing comes to my mind right now, and I don't reproduce it.  If you could get debug logs from a period when it happens, that would be nice.

There's certainly no such optimization, so it's most likely some kind of bug.  Possibly in the dns64 module, as that's a more fragile part than the core.

--Vladimir