Found a solution/workaround in the meantime; haven’t had the time to share yet.
Copying from one of tracks 1-8 to a midi track works, if the instrument on the source track is a midi or synth instrument - remember: tracks 1-8 are not limited to sample instruments, you can assign midi/snyth instruments to track 1-8 
So if you want to duplicate a note sequence from an audio track to a midi/synth track, do the following:
- Copy the sequence to another audio track. If your current pattern has no more free audio tracks, use a new pattern (remember to adjust the pattern length if needed)
- Change the instrument on the copied sequence to midi/synth instrument(s).
- Copy the modified sequence to the desired target midi track
- Delete the “intermediate” track
Worked for me. But I’m not sure, if possibly some FX lane settings might impede.
If you just want to move from audio to midi and not retain the original sample sequence, just change the instrument on the source track to midi/synth, copy to target and delete source.