Hi DiglyD
Thanks a lot for your very valuable feedback, it gives me a ton of good ideas to dig into!
To explain how the midi export works, if you have for example a Guitar 1 on instrument 1, but in pattern 2, a Synth 1 on the same instrument, the midi export will create 2 midi tracks: track 1 / Guitar 1 and track 2 / Synth 1:
I can't reproduce your bug. I open a template in Reaper, with preloaded instruments, and I drag the midi file into Reaper DAW. When I hit play, all the instruments are playing: