Hey there! Can you make sure that the MIDI track doesn't have any plugins with audio output on it? i.e. the "volume meter" on the track should be dots that show MIDI velocity, instead of a VU meter.
Here's a screenshot of an example, where track 1 is a MIDI track without any plugins (and routes successfully), track 2 is a MIDI track with a plugin (and has the same "Track In" output channel you're describing), and track 3 is an audio track with Melodrumatic on it:
Let me know if that works for you!