There is an issue with the improved stack mechanic for neutral creatures: it works on one map but not on another.

The number of stacks directly depends on the strength of the army you are attacking with. If the armies are roughly equal, the number of stacks and the guard shift (GS) will be the same. If the strength differs too much, yes, it can happen that the neutral creatures stand in one stack, and there will be no GS.
+ even if the hero is 2 or more times stronger, the neutral creatures may not stand in one stack. I don't remember the exact percentage, but there is a certain chance that the neutral creatures will split into 2 or 3 stacks.