Beside the fact that I love using Cubase, I have been regularly stumbling upon a quite old and very annoying bug, which is still there (I use the very last version of Cubase 11.0.41 on Big Sur 11.6.2). By the way, I remember having it already on Cubase 9, if I am not wrong.
As I am rather a patient and understanding kind of person, I am not used to post messages in the forum. Indeed when I discover a bug, I know that it will be fixed in the next release most of the time… and I can wait for it. But to be honest, I cannot understand why this bug was not fixed before as it is very disabling and easily reproducible.
Because I think a video speaks more than words for this kind of thing, I recorded my screen and make the result available on a famous video platform. I hope it can be helpful. Here is the link: - YouTube
From my own experience, it seems that when two audio events on two separate tracks have the same size and position (i.e. exactly the same start and end points), then they turn on to be linked like hell for ever… so linked that even death cannot do them part: when you delete/move/select/unselect/you-name-it one, the same happens to the other.
In my example, I edited the track below to fit the size of the track above, so in this case only an “undo” operation can save you from the “ineluctable” at least if you notice it early enough (fortunately it was my case here). But if you go on with your work carelessly, “undo” just cannot be the solution as it could compromise hours of work that you did in the meantime… or you can just think of the case when you saved your work in the meantime and quit Cubase, then the bug will be irremediably there the next time you open it. In such damned case, there is still a solution to escape from the “linked selection” bug: clicking on one of them will select both of course, but double clicking will open only one sample editor. From this window you can move the event start and end markers, thus breaking the link (you’re welcome). This operation works fine but is EXTREMELY cumbersome, especially when a hundred of events are affected (you can easily imagine why…). This happened to me a couple of weeks ago
I would be really grateful if you (the Steinberg team) could put some effort to fix this bug. I would be also of course glad to help if you need further information.
Thank you very much for your answer (and sorry to reply that late).
Tracks are indeed inside a folder but with group editing disabled. You can actually deduce it from the video, because as soon as you cut one of the events so that the end positions of both events (on different tracks) coincide, then the tracks become linked. Undoing the operation, or moving the end point of one event inside the event editor) is enough to break the link. This is what I tried to show in the video. This behavior would not happen if group editing was enabled I guess.
I don’t remember about the Fade-out, but I would say that it’s not that important as it was just an example. As soon as the tracks are linked, ANY KIND OF MODIFICATION you do on one track is simultaneously applied to the other one. It can be fade out changes, but also selection, move, delete, you-name-it operations.
The difficulty with this bug is that one cannot always reproduce it, even though it appears quite often: my last two project sessions have unfortunately suffered from this bug. There are probably specific conditions which provoke it, but I obviously do not know them all. I can say that both tracks are within a folder track. Then I cut one event on one track twice on the left and on the right, defining new begin and end points. Next step: I enable (with “j” key) the “snap on events” function so that I can cut the audio event on the other track to fit exactly the event position and size from the first track. What I can assure you is that these conditions are NECESSARY (probably not sufficient though) to reproduce the problem: position and size must perfectly match. By the way, this also happens with MIDI or instrument events (not only audio ones).
so linked that even death cannot do them part: when you delete/move/select/unselect/you-name-it one, the same happens to the other. 
(fortunately it was my case here). But if you go on with your work carelessly, “undo” just cannot be the solution as it could compromise hours of work that you did in the meantime… or you can just think of the case when you saved your work in the meantime and quit Cubase, then the bug will be irremediably there the next time you open it. In such damned case, there is still a solution to escape from the “linked selection” bug: 
clicking on one of them will select both of course, but double clicking will open only one sample editor. From this window you can move the event start and end markers, thus breaking the link (you’re welcome). 
This operation works fine but is EXTREMELY cumbersome, especially when a hundred of events are affected (you can easily imagine why…). This happened to me a couple of weeks ago 