What you are trying to do is easy in a DAW. In BiaB have the piano play for the whole measure. Drag and drop the tracks into a DAW. On that measure use volume automation to fade the piano, then at the start of the next measure sharply bring the volume back to normal.


OK, a random thought;
Why does toilet paper need a commercial? Who's not buying it?

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware