Haven't looked at the script code to see if it's using the group object bu I assume it is, and as I recall unless that was changed since I last investigated it long ago, the group objet has a game-imposed limit on how many members the group can actually contain.
So this means it would automatically remove or replace a member at probably the 1st or last member index with the next one added (spawned). The result of that means the member removed from the group is no longer being controlled by the games group logic which does many things including making it not attack other peds in the same group.
This is just a hunch as to why they start fighting each other when you go over a certain number. Might be wrong since I haven't seen the code to confirm the way it's setting them up.