Im sorry to hear about that experience, but I can’t say I’m surprised. I’ve generally found the quality of con GMs to range from terrible to poor with the occasional bright spot.
Of the six games I played at Garycon this year, two were hall of shame bad GMs (one muttering as he read not only boxed text but also the rest like about secret doors and traps and getting pissed when we “found” them, the other kept inserting “superior” 3e rules in a 5e game and then belittling younger players who questioned rulings that invalidated their class features), two were simply bad, and two were pleasantly meh.
I only played three games at Origins this year, and while the Star Wars GM was pretty poor, both D&D DMs were actually good, one bordering on excellent. I’ve literally never experienced anything like it before (I asked my buddy if it actually happened or if I’d had a stroke).