This might not be a problem and just is as intended, but im only referencing my own memory of only one encounter to them. Spoiler ahead which might answer your confusion about your experience:
One of the Minotaurs is wearing the "Amulet of the Unworthy"(Resistence to Slashing damage)
As you wrote you used Cleave with a Two-Hander i assume it was a Slashing Weapon like the Greatsowrds.
Thunderwave is another thought. As Minotaurs are big Creatures they have a lot of Weight. By that i mean they cant be toppled easily, because theay are to heavy and are like a rootet Tree, not easily staggered.
But im not sure about this Just a general thought which can provide an answer. Im not totally familiar with all 5e Rules.
Just common sense using.
I believe this to be the reason, and the minotaurs not being bugged(the Bulette on the other hand ;P).
You can pull the minotaurs easily by throwing a torch into the fog, it's not super hard when you aggro with a tank-y character, and have your other party members rain death on them
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