Reaper hung midi note issue threads
This is by no means a perfect or complete list and I slapped it together in about five minutes of searching on the Cockos Forums and /r/reaper for "hung note" and "note off". I kindly ask that you just use this as initial evidence of a real problem, then do more comprehensive research to sort out the variants of it, any false-positives and user-error, and/or bugs that have actually been fixed historically. For instance, some people report the "all note off" action working to silence the notes, and others (myself included) find that action does nothing to help and end up doing crazy hacks. Check out the first link which is a thread around an entire video one guy made on how he works around the problem. For me and for others, that hack doesn't work.
On the Cockos forum there are lot of old threads on this bug that have survived through the years with folks continuing to chime in with recent versions, suggesting this is an old problem that still persists.