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.