Hi Ben,
Okay, first of all, let me apologize of having seemed rude, but I was a bit disappointed since there is still no answer to our comments posted two years ago in the topic "Prophet V 2.0 & Mainstage 2.1.2 - Notes Stuck" (
http://forum.arturia.com/index.php?topic=4656.0).
So, after further investigations, it seems this issue with Matrix can be solved by reinstalling Studio One (or maybe at least rescanning Studio One's plug-ins folder). Here are the steps I've followed :
Note : I didn't used any MIDI controller for the entire procedure. Notes were written by hand via S1 piano roll.
1. Create a new Windows (admin) user
2. Close the actual Windows (admin) user session
3. Open the new Windows user session
4. Open S1 (now S1 is scanning his plug-ins folders) and create a new song with one or more Matrix instances
5. Stuck notes are gone
6. Save the song
7. Close the new Windows user session
8. Open the old Window user session
9. Open S1 and load the previously saved song
10. Stuck notes are back
11. Uninstall S1 and reinstall it
12. Open S1 (now S1 is scanning his plug-ins folders)
13. Load the saved song
14. Stuck notes are gone.
Conclusion : this issue may be caused by installing Matrix's update without uninstalling the previous version (as I've done). In this case, S1 doesn't scan the new version and may "consider" it as the old one and therefore cause the "stuck notes" bug. Of course, I have no certainty ; it's only the hypothesis that seems the most plausible to me.
Thanks for your support.