That one complete fan fiction anime film was made only so that people's standards are low enough to love it simply for being true to the lore and having the original film cast reprise their roles.
Also, Kirkman genderbent the main character of a spinoff comic, and one of the staff started schizoposting, calling the entire fandom misogynistic for the dumbest reasons ever, so that may have also pissed them off.
Invincible has always been pretty ass, but this second fight as well as the garbage filler was bad enough to actually piss off the part of the fandom that defends everything.