Dave, does the Yamaha have a midi channel assigned to each part ?

Left side, right side parts. Just about every midi instrument does.

Then you should be able to defeat cc64 (hold) for the channel that uses bass.

I could not find a midi chart for the piano.

I found the reference manual for midi.. you should be able to turn off cc64 for left.

https://usa.yamaha.com/files/download/other_assets/2/1209032/p125_p121_en_mr_a0.pdf