When I use Reaper back in Windows OS, what I encountered was all midi tracks were armed, and whenever I play a note on keyboard, all midi tracks were playing that note on each VST plugin they currently are running. Fortunately, there was an option that is “arm only selected track”. And that was it, it solved my problem in Reaper.
I’m having the same issue in Ardour 6 (running on Ubuntu Studio 21.04). I added a screenshot down below, as you can see, I have 2 midi tracks each are running separate VST’s. When I select one of the midi tracks and play a note on the keyboard, the other (unselected) midi track also plays that note. I dig everywhere on the manual, preferences etc. but nothing helped me to fix it. I can’t find any topic on the web. So I decided to create this topic.