Alright so I've had this problem in the game for some time. Thought it only applied to Joy Joy Balloon but it happens with Hotaru's stances too..
It seems that at times, there are instances where I can't even stay in one of the said stances. The animation for entering the stances only stays for the first frame then automatically cancels itself out, without pressing anything. This stops when I do reset the stage though.... is this an intended mechanic or is there something wrong with my controller?
I believe it has something to do with the actual button inputs. I remember having to hold buttons for the stances to remain and I could let go and do whatever afterwards. I'll do some testing today.