Make sure Ripple All Tracks is on. Have two clips one on top of the other. On the bottom clip create a transition. On the clip above, split the clip anywhere before the transition below. While still on the clip above, go anywhere after the transition and ripple trim the clip in (Shift+I) so that the transition below is deleted. Now undo. On the track below the clip that was to the left of the transition will be deleted. Demo.
I wonder if this is what has been biting me.
I had noticed that if I accidentally left the Ripple buttons on, and went on with an edit similar to what is described above, if, when I realized my mistake, I tried to correct it with Undo, I was sometimes left with a missing clip and shifted track.
It has been frequent enough, and serious enough, that if I detect that I have made that mistake now, I don’t even attempt Undo; I go back to the most recent numbered Save As and redo my work since that checkpoint.
This is fixed for the next version 21.04