Hi,
I can’t see any reason in your screeenshot that your multi instrument would be retriggering, but it’s difficult to say for sure without access to your project. To start, so I can test issue on my end, what versions of FMOD Studio and FMOD for Unity are you using?
As a workaround for the moment, you should be able to force monophony by selecting the multi instrument, opening the Trigger Behaviour menu on the right side of the Playlist, and setting the polyphony to 1 with the Stealing mode to “None”. This will ensure that the instrument will not retrigger until it has stopped playing completely.
An empty audio clip can work, but you can also enable “Probability” in the Trigger Behavior menu, and use the “Chance” setting to set the trigger chance to the desired value. If you set this to 50%, the instrument will only play 50% of the time when triggered.