After some investigation into the code of the Digitakt concerning the neighbor track functionality, this is how it works:
Audio track 1 is the neighbor of track 2 and it is also the neighbor of itself (making the NEI conditions work like the PRE conditions on track 1).
Audio track 2 is the neighbor of Audio track 3.
Audio track 3 is the neighbor of Audio track 4.
Audio track 4 is the neighbor of Audio track 5.
Audio track 5 is the neighbor of Audio track 6.
Audio track 6 is the neighbor of Audio track 7.
Audio track 7 is the neighbor of Audio track 8.
Audio track 8 is the neighbor of MIDI track 1.
MIDI track 1 is the neighbor of MIDI track 2.
MIDI track 2 is the neighbor of MIDI track 3.
MIDI track 3 is the neighbor of MIDI track 4.
MIDI track 4 is the neighbor of MIDI track 5.
MIDI track 5 is the neighbor of MIDI track 6.
MIDI track 6 is the neighbor of MIDI track 7.
MIDI track 7 is the neighbor of MIDI track 8.
MIDI track 8 is the neighbor of no track.