SOLVED: I created a video group, added my layers to it, and now I'm able to move clips from one layer to another.
PS 23.4.1 on MacOS 12.4. Using video timeline. When I try to move a very short clip from one layer to another, for animation, I'm unable to do so--the clip at the target layer jumps to another layer, PS doesn't seem to want me to have more than one clip on a timeline layer.
After selecting the move tool, I can see that "auto select" is unchecked (I've also tried checking it). I don't have any layer groups yet. No layers are locked.