Trippier was stepped on before the pen tackle, which could have been awarded as a foul, Trippier then fouled the Fulham man and I do think that merited a pen. But the decision in the first place was a weird one and it did need to go to VAR.
Then, Mitrovic scewered the penalty and that resulted in the ball hitting his other foot and going in the net. I read a quote saying that the rules on penalties are quite simple, that doing something like that results in a spot free kick to the opposing team. It just almost never, ever happens, so of course people are up in arms. It's another case in football of the ones being punished doubting and wanting to change the rules.
If Mitrovic moved to early, it would have to be retaken, and that is were the "confusion", better known as bitching, is coming from.