I've noted this in the campaign as well, I take 3-4 shots at the A.I and it just won't die until a 5th shot.
In the campaign I've decided to set it at the lowest difficulty, then set the enemy A.I skill value to = 0.00 and they STILL just stand there, look at me for at least 1 second *this happens while I am prone as well.* then somehow just one shots me. This issue really just infuriates me greatly and it's just pushing me farther away from trying to enjoy the campaign entirely, this must be resolved soon!