[...] I’ve found that a specific external domain that is important to us isn’t able to resolve successfully. I’m not sure if there might be other domain with the same problem. Any suggestions would be awesome.

I can't make mtn.com fail.  Does it still happen?

Usually it's best to get verbose logs for the failing request.  If it happens rarely, you can e.g. restrict the verbose logging just to some names and only on failure states: https://knot-resolver.readthedocs.io/en/v5.2.0/modules-policy.html#policy.DEBUG_IF

--Vladimir