Glyn Barnes
I had a similar issue using NI Komplete Kontrol. MIDI out was enabled in in KK plugin so all input into the KK tracks was being echoed to all omni tracks in focus.

It was perplexing at first until I realised what was happening.

I think that's it.
 
Scook actually mentioned that the 'Enable MIDI Output' feature could be the problem, so shout out to Scook on that one.
 
Thanks for chiming in, Glyn! I'm troubleshooting it now but it's a task to do! There's no way to disable the 'Enable MIDI Output' function after inserting the instrument, so I have to re-insert the instruments. After doing that, I tried to re-route audio and MIDI I/O to the instruments I just inserted to replace the ones from before but, it turns out that in the Inspector pane for instrument tracks, the Audio and MIDI tabs at the bottom both only show MIDI INPUTS! So, now I have to separate all the instrument tracks into audio and MIDI tracks to be able to change the audio I/O......................!!! Lemme add that de-selecting MIDI/Instrument from the Strips menu in Console view STILL hides BOTH MIDI AND Instrument tracks! 
 
Anyway, thanks guys!

scook

Samuel540
There's no way to disable the 'Enable MIDI Output' function after inserting the instrument, so I have to re-insert the instruments.

Try right-clicking the synth in the Synth Rack to bring up the context menu. Then disable the feature there

It's annoying that a instrument doesn't tell you it's outputting MIDI - Cubase has a little icon in the right hand side of the rack  ->] [-> which glow when active. Would be handy to have something in SPlat so we could see if MIDI is hitting a track and MIDI is being sent from it.

Well, guys, it seems the problem has been fixed!
 
Solution: Simply disable all 'Enable MIDI Output' on the VSTs (from the Synth Rack...) and you should be fine; it won't matter if your MIDI inputs are on Omni.
 
Thanks to all who contributed to the thread.
 
Until next time............... shouldn't be long.