If the plugin doesn't flush its buffers when it receives a stop message from the host, any tail audio will be incorrectly included in a render, unless the user waits for the tail to stop every time.
Massive X should clear its internal buffers when it receives a stop message.
Please do this on any stop message, regardless of whether the host was playing.
Interestingly, Kontakt 7 does not have this issue. Maybe this implies different internal dev teams that maybe should work together?