Thank you for sharing the solution!
All instruments on in an action event are async: FMOD Studio | Working with Instruments - Synchronous and Asynchronous Instruments.
Programmer instruments should stop even if they are in action sheets:
I suggest making sure the programmer sound is set up as the following: Loop region and programmer sounds - #2 by Connor_FMOD
Hopefully, it will avoid other unexpected behaviour.
Please let me know if there is anything else we can do to assist!