Actually, it does still do it on a couple of patches. Maybe it has something to do with the type of sound and a phaser causes some kind of cancellation. I remember once having a guitar track recorded in stereo with a phaser on it and when I set the monitoring to mono it went truly silent, not ghostly of faint, but silent...
Can one of you with more knowledge try to replicate or maybe explain to me what is going on if it is natural time-based phase cancellation and not a bug?
"Factory Content/Keyboards/80s Synth Clavi XS"