After enabling a Disabled Track, midi Inputs are lost if that track was Duplicated from another Disabled Track.
Cubase Pro 8.5.15
Steps to reproduce:
1-Create an Instrument track with your favorite Instrument (Kontakt in my case)
2-Make sure the midi input is set to: “All MIDI Inputs”
3-Disable the track
4-Duplicate the track
5-Enable the newly duplicated track.
6-Midi inputs will be missing.
7-Enable the original track
8-Midi inputs will be missing as well.