A rehearsal mark behaving weirdly

I have already solved this but I don’t know how it happened, and I suspect that what I’ve seen shouldn’t be possible, so I’m sharing.

Today I that one (and only one) of my rehearsal marks in my score appeared on all staves even though I have them set to only show above the top staff. In addition, the extra marks were placed on top of the music: inside the staves and further right than the top (properly placed) one. Selecting one of them, I confirmed that no option in the lower panel was selected. Dragging the mark and then resetting the Start offset caused all the marks to jump to reasonable positions.

After searching a while for a way to get rid of the extra marks, I eventually tried changing the sequence type for the misbehaving rehearsal mark. That fixed it, and after resetting it everything looked the way it should.

(Dorico 6.2.10.6140, on Mac.)

It has been reported sometimes. There are several ways to get rid of these intrusive rehearsal marks, and it looks like you found a new one!