I encountered an issue recently where, while I was playing notes while switching between mono- and paraphonic mode, the Pro 3 got into a weird state where one note was always stuck on, unless I played that note, which would turn it off while I was holding it down. I had to restart the Pro 3 to get out of that state.
Probably not related to what you're seeing, and I only encountered this once. Sharing in case there's anything that sounds familiar.