cubase will only remember ( record/save ) controllers etc ie note on/off..velocities.midi channels..
it does not know how you setup your digital piano
if you use native instruments ( vsti )this problem does not occur as everything is within the
computer
assuming you recorded 3 midi tracks
ch1 for piano
ch2 for choir
ch3 for drums
then you must setup your piano so midi channel 1 has the piano assigned to it
midi channel 2 has the choir assigned to it
and midi channel 3 has drums assigned to it
you can usually save and send patch changes to your keyboard....but all keyboards are slightly different
so check your documentation.it seems you're sending all your midi to 1 channel which has a piano
assigned to it.........or midi ch 1/2/3 have a piano assigned to them
a good rule of thumb is when you like a particular part you've written on midi.. say a piano part
record the audioonto an audio track in cubase ...this way you will always have that sound
regardless of what happens to your keyboard...still keep the midi though in case you want to edit it at
a later date ( notes/sound etc )
a good starting point is... after deciding a tempo record a basic drum track..so you dont have to use
the onboard metronome ( except for count in )
render it to audio ( you only have to record say 4 bars ) then copy and paste the length of the song