What happens if you stop the playback after you do the fill, mute the filled track that caused the change and then start playback again? Is the melody of synth 2 still changed? Or does it play the expected melody?
If the melody of synth 2 then plays what you expect, what happens then when you unmute the filled track?
I’m asking because it sounds like it may be related to some of the weirdness I’ve reported as another bug.