Easiest way I do it:
Put the MIDI track under the Synth Out track (icon is the midi plug + keyboard).
Click/hold on the Synth Out track number (highlights in blue)
Drag down over the MIDI track number, both should now be highlighted.
Head north to the view/options/tracks/clips/midi/V-Vocal - click the dropdown on Tracks.
Click Bounce to Tracks
I keep entire mix since it's only that track - track only (sometimes) gets that 1-line like you're seeing, so I just go with entire mix.
Prob a reason for that, but I just go with entire mix
Oh, and I rename the track to whatever the MIDI track was