RuntimeManager allocates memory OnGUI even when Overlay Debug is disabled. I commented out the code, and it stopped to allocate memory every frame. Unity 2018.3.12f1 and FMOD 1.10.12.
Rect windowRect = new Rect(10, 10, 300, 100);
void OnGUI()
{
if (studioSystem.isValid() && Settings.Instance.IsOverlayEnabled(fmodPlatform))
{
windowRect = GUI.Window(0, windowRect, DrawDebugOverlay, "FMOD Studio Debug");
}
}