Thanks, @benwiggy. (I know about that third system).
It was actually the staff attached text (Kortstokkhæren:) which caused the reversal mark to position high above the system:
Setting the position of the text to “below” in the properties, solved the issue:
In my first post, I had manually dragged the text and the rehearsal mark, and I thought that this would make Dorico recalculate the vertical spacing. I guess this thread is debating the same issue: Suggestion for Rehearsal Marks (placement with other objects)