I noticed most of the time the zombies are just holding their melee weapon and standing in a group getting shot by my players instead of throwing grenades or shooting the guns I gave them.

I am having this same issue on v3.4.3 and it seems they will shoot, but only when they reach melee range. Wonder if it would be possible to just make whatever parameter controls that configurable like number of meters they start shooting or something, just spitballing.