Over a week so far of pet farming, AH has almost 2 hundred pets on my server, checking multiple times a day every single day, not a single one of the 3 Naxxramas pets has appeared
I ran it 4 times, no pets dropped.
Have multiple friends (several dozen, achievement discord) that got over half the pets needed for both Raiding with Leaches achievements, no Naxx pets dropped for them either.
Myself and a friend have run 6 runs over the last few days for 0 pets as well.
So if everyone’s numbers are correct, 18 runs, 3 possible rolls per run that’s 54 possibilities and all are supposed to be a 25-30% droprate. That’s insane bad luck (in fact, calculations would say 99.9999820812% that you get a drop over 54 instances at 25% droprate).