I haven't seen this reported anywhere yet, so apologies if it is already known. When I got to 21 reaper synths and it created the 2nd stack, trying to use the second stack of 1 actually deleted one from the stack of 20, leaving a stack of 19 and a stack of 1. I then used a few more just to make sure I was trying to use the stack of 1, and it continues to delete from the larger stack, so there seems to be no way to get rid of the 2nd stack unless you delete all 20 of the original stack leaving you with 1 left. This is not the case with any of the other synths, only the reaper synth.