Theres a bug in 9.5 and 9.5.1 in the mixer undo feature. It just randomly stops working. But from trying to narrow it down, it seems to happen when you move a VST effect from one insert slot to another, or when a VST effect is removed. I cant be 100% sure though that this is the only cause, its just Ive observed it happening when these actions are executed. A resolve to this problem is needed.