I'll try digging out the side. It's actually a peninsula tile (coastlines mod) so maybe they can only leave through the north, even after digging to the side.
What I did in the meantime was to destroy the guests in dev mode (sorry friends), change the beds to normal beds, and remove the mod from the load order. Then I loaded the save again without the mod, saved, and put the mod back in the load order and loaded the save. This seems to have worked. A quick test (spawn visitors and then ask them to leave, which was causing the same issue before) shows that the problem is fixed. For now.
What I did in the meantime was to destroy the guests in dev mode (sorry friends), change the beds to normal beds, and remove the mod from the load order. Then I loaded the save again without the mod, saved, and put the mod back in the load order and loaded the save. This seems to have worked. A quick test (spawn visitors and then ask them to leave, which was causing the same issue before) shows that the problem is fixed. For now.