I can’t be sure without seeing your events, but it sounds like event instrument in the parent event might be spawning a new instance of the referenced event every time it loops, instead of playing one instance of the referenced event continuously. If so, this is most likely happening because the event instrument is inside a loop region that’s untriggering and triggering it once each loop. Try stretching out the event instrument to be slightly longer than the loop region, and see if that helps.
If that doesn’t help (or if it makes no sense in context) then my guess about the structure of your event was incorrect. If so, please post some screenshots of the two events so that we can see how they’re structured.
If you’re unable to upload screenshots to this forum, perhaps you could upload an FMOD Studio project that exhibits the issue to the uploads section of your profile page?