The pan-control on a MIDI-track is just a knob controlling the software synth's Midi Controller Message 10. Most synths have this parameter linked to the pan position, but it's not hard wired to this. (I haven't used True Piano much, but I'm pretty sure it uses CC10 for pan, though).
The pan knob on a synth's audio track is a direct control over the pan position of
all the sound comming through it though and with this you should be able to have total control over the pan position.
If you can't pan the sound with the audio track, something isn't right and the reason for this can be several different things. I find it strange that it works for you though when you pan both the MIDI and the audio track..
What happens if you pan the MIDI track hard right and the audio track hard left?
Some things you can check:
- Have you any VSTs in the FX-bin (on the audio track or a bus that the track is sent to) that is mono or set to mono?
- Do you have any sends on the audio track set to pre fader? This will send the audio from the track to another bus before it runs through the pan module of the track.
- Did you output the track to a bus that is set to mono?