I did try to analyze the issue and it seems like authorative nameservers aren't answering all queries. And there isn't a system in what is being answered. I probed from three different Zonemaster websites (DNS analytics). Two of them report UDP connectivity issues on some of the nameservers.
That is also why you'll see the one dns server not resolving your a record, but the other will.
Curious, I was trying to get to one specific site (secure.tsp.gov) and was getting DNS error nx found (using 220.127.116.11 as primary). Change to 18.104.22.168 as primary and it now works. Maybe the recursive lookup or maybe TSP changed recently. Anyway it is fixed for now, but wonder why the two respond with different results.