It’s probably not sufficiently clear from the way the option is described, but the idea behind this option is that if you end up showing rehearsal marks on a staff in the middle of a system, the offset should be applied to avoid a collision with the systemic barline; it’s actually not supposed to take effect when the rehearsal mark is above the top staff or below the bottom staff of the system, so in fact the bug here is that it applies when there is only a single staff.