I can repeat this. The pattern, at least here, seems to be this: if a track is active, and you select 'none' in the dropdown list, it will show 'omni'. If you select 'none' from a non-active track, it will show 'none'. If you then make that track active and select 'none' it will change to 'omni' AND it will change the previously active, previously 'omni' track to 'none' even though you haven't changed anything there.
Additionally, if I select a track with input set to 'none' in this scenario, I can play it. These are instrument tracks, but I tried splitting them up, and also assigning a new MIDI track to output to an existing synth, and it seems to be the same.
This does seem a bit strange to me, but I'm not enough of an expert to say whether it's a bug or some mysterious functionality that I don't quite understand. Anyone more informed have any thoughts?
I guess for the moment the workaround is to make very specific MIDI input selections if you want explicit control, and otherwise ignore this mysterious omni/none discrepancy, since they appear to exhibit the same behaviour. It's counter-intuitive, to say the least...