I’m very glad to find this thread and your notes on it.
I was just having this problem in Cubase Pro 11.0.20. Specifically, I’d just enabled two instrument tracks (both with instances of Kontakt 6 with Strummed Session Guitarist 2 on them), after having them disabled for some days while working on other parts of my arrangement. But I was getting no sound from the MIDI playback on the newly enabled tracks. Nothing was muted (as far as I could tell anyway). Playing the instruments live worked fine. Selecting a note in Key Editor also made sound, so the connection between the track and the VST instrument was clearly working, even if no sound happened on playback (and the Kontakt keyboard didn’t show any activity on playback, either, so it was an issue of the data not being sent from the clips, not a playback issue within Kontakt or a disconnection at the track level; I’d also tried explicitly muting and unmuting the clips using the Mute tool earlier with no luck).
The suggestion you made regarding selecting the track and using Edit/Unmute was key to getting sound back. However, Edit/Unmute was not available at first, so I had to do Edit/Mute first, then Edit/Unmute. After that, not only did the instrument track I’d selected play back, but the other one that was having the problem did as well – i.e. I didn’t have to do the Edit/Unmute separately to get it back to life.
One possible clue here. It is conceivable I’d muted the two tracks in question prior to disabling them. In particular, I had a deadline on a song demo, so I was trying to get to some stripped down version of the project that I could pitch, and these two tracks weren’t far enough along to include by the deadline, so I disabled them to take their load off the system. I may very well have muted the tracks before doing that to see how the project would sound without them, though. So, maybe the issue (at least for my case?) might be that, if tracks that had been muted when they were disabled got (re-)enabled, the Mute button at the track level does not show up as muted, but somehow the clips on the track are in some muted state, which the Edit/Mute then Edit/Unmute clears. If so, that would clearly be a bug in Cubase.
I’m just glad I was able to restore the tracks to making sound without having to create new instrument tracks and move the clips as that would have been a pain in the context of these specific tracks between tweaks I’d made within Kontakt and the tracks’ being multi-lane to separate out key switches and CCs from the chord data (notes).