I just tried updating to 2.02 instead of 2.03 and the stuttering issue is there as well.
Another data point is that a mission with way fewer vehicles on it - and so way fewer events - does not seem to stutter.
edit: I tried putting a bus instance limit of 4 on the engines and it seems to have fixed the stuttering. It’s not a solution unfortunately (I just posted Group bus instance limiting for continuously playing events) and it’s still a mystery to me why this is working in FMOD 2.01 but not 2.02/2.03. It makes me a little nervous to update even if I do get the custom limiting code to work.