+1 this also happened to me
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.
This doesn't explain them taking 0 damage from Cleave (I believe that the game didn't even roll a cleave attack roll or damage but I might be misremembering...). It even more doesn't explain
neither minotaur taking damage from Cleave.
Thunderwave not affecting them because they are big follows common sense. But it should then be reflected in their immunities when you examine them.
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