Ron, I had the same issue on my Tyros 5.
I discovered that if the split points for the Style and the Left Part are not the same, then the left part is muted when playing a style with ACMP on (even though the keys pressed are within the zone for both split points).
So, to be able to play a left part voice when playing a style, you need to make sure that the split points for both the Style and the Left Part are set to the same note.
Regards
Roy