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.)