Hey there. When people have reported something similar, it turned out that they'd misremembered/were actually playing on Normal/were thinking of the Zydron boss encounter which is not what the triumph is looking for. Best thing to do would be to wait for Vex to be round 1 or 2 for Legend Dares, run the activity, and then if it didn't count, you've got simple proof. Otherwise someone would have to dig through your game history and cross reference every instance against the weeks the Minotaur encounter has been available.