i made 2 custom arches and the eyedropper tool works on neither of them, while it works just fine on anything else.
In case that matters, it's clones of doors that I changed to use the meshes of matching arches. Anyone an idea what's wrong there?

If build mode objects act the same as buy mode objects - I'm wondering if the bounding box is the issue. Try going to the Model tab in the GMDC and clicking the "clear" you'll see to your right. Then go to the Groups tab, click on the subset(s) then on "add to bounding box". Commit and save. Since this is a door/arch, you'll want to do that for both GMDCs (north and south).

P.S. No need to do this to the shadow if your mesh has one, just the actual door/arch.

They don't have a GMDC because I just pointed them to that arch mesh anyway, so I assumed I could delete it and it seemed to be fine when I tested... that was probably not a very awesome idea, I'll find the backup and try that out.

I actually found the issue because I noticed that an earlier version was working. It happens when I recategorize the former door/now arch to show up along with the arches. Does someone know why that breaks the eyedropper tool thing or do I simply have to leave it in the door category?