There is an elite unit that can be present in wave 4, General Kalani, he limits the amount if droid stack that can be removed in one turn.
I've added a screenshot of his abilities.
I've had the same issue - but for me, it seems somehow connected to using JKLS. When I use him in the for any of the Jedi battles, the droid stacks don't reduce with assists, counter attacks, etc. - only the single first hit.
I had someone point out that not only does General Kalani limit the amount of stacks that can reduce on B2 to 1, but he also adds 2 with every basic attack. Both General Kalani and elite b1 command Droid have counter elements in their kits, so maybe it's reducing and then adding back from counter attacks?
