Why was Mahomes so angry?
Because the refs held it as a screw you.
Refs could probably call holding on every play, but don't.
Just because you do it, you don't get warned, and it doesn't get called every time doesn't mean it's not a penalty. Yeah inconsistency from the refs sucks but it's part of the game for every other team, not just the Chiefs. And the flag was on the ground before PM even threw the pass to Kelce so it's not like this was some dastardly caper designed to reverse an amazing KC touchdown.