It can happen due to "Selection follows playback" and also "Playlist auto scroll" option (it scrolls the playlist so that the playing track is visible). So when dragging, it may cause conflicts. It seems rare though, I just tried couple of times and it seems to still be working fine.
Most of the time, and even all the time for me: a selection of additional tracks. See the video file "Sliding."
In the video, three tracks move simultaneously while only one track is selected. For your information, it's the same selection before and after the transition. That is, I left my finger on the mouse button.
Rarely: another track is played. I had more difficulty reproducing this behavior (PlayAnotherTrack). I think it was a matter of bad luck: I released the mouse click right at the transition and pressed it again right after. Perhaps my mouse has a certain sensitivity that caused a second click. But I didn't feel like I was doing it.
In any case, it's surprising to see the playlist's appearance change during a transition: we have reflexes that can create unwanted actions.
Disabling the "Selection follows playback" option allowed me to no longer experience the behavior of the "Sliding" video.
Assigning a different behavior to the double-click allowed me to avoid unpleasant surprises. Even if it was rare. It's a safety precaution for me: I may have inappropriate reflexes at the wrong time or perform actions too quickly.
If there's a frequent need to drag tracks while playing, probably those two options should be disabled.
For me, only one of the two: 'Selection follows playback'. Even if I don't often change the order of the tracks.
The other option, "Playlist auto scroll" enabled alone, didn't create any strange behaviors during the tests I conducted.