Today I tried to reproduce the problem with the invisible attackers.
So I Hit an zombie with an Axe. First time the infected fall down like a tree, and crowch on the ground, then he glitched thru the ground and keep moving/attacking me. So it was not viewable, but not really invisible :-)
I reproduced it two times in 2 other areas, and it was the same. Infected glitched in the ground and crowched underneath the ground and attack.
So you maybe better shoot into the ground to finish them.
Hitting the ground with an axe will not work. Collision request will work for the axe TO the ground, but not for the infected OUT OF the ground.
So please check that.