You might need to check using the style editor.

Pick STYLES>EDIT CURRENT STYLE.
The pick PAT and you should see a list of instruments / tracks.
Also there should be a box below that allowa different instruments in the a and b parts. If this is checked you might find that two different pianos are being used.



I have never been able the change the instrument at the song level if they are selected this way.

I use 2013, so it might be different in the newer versions.


BIAB for Windows Version 2019 (643)
Windows 10 (1909)