At this point I pretty much think this may be the exact case - and it may work in reverse: “since ns2 has AAAA listed then it’s an IPv6-capable deployment and we are allowed to fall back to IPv4, but since ns1 lacks the AAAA record, then we won’t even consider falling back to ns1 if ns2 is down”.

Or maybe another scenario:

• let’s check ns2’s AAAA record
• oh it’s down! Let’s check ns2’s A record
• it’s down too! Let’s switch to ns1…
• … but at that time the DNS resolution timeout expires, since we took too long resolving the records; and on the second try, no one remembers that ns2 was down, repeating the cycle