I think this is a bug? After splitting a stereo track in half and then splitting into L/R mono tracks (now 4 tracks), and then doing a shelf-filter adjustment on one of the tracks, a re-join is automatically being applied to that channel. This might be happening more broadly. I just happen to be using the shelf filter when I ran across this.
The specfic details - I’m doing a stereo cassette capture and then minor audio adjustments. I split the captured track at the mid-point into side-1 side-2, then split the whole thing into L/R mono. Then I apply a shelf filter adjustment to side-2 R track. When It completes, it’s automatically rejoing the side-1 and 2 R tracks. (The L tracks remain unaffected.)