I believe that portal separated the upper floor from the lower floor in that cell. I'm grasping for straws here, but I have 2 theories, lol.ġ – this is somehow related to a portal marker I removed. No, I don't think it's the kit piece either as it's not like that in the vanilla cell. So deleting yours and re-adding it would not solve the problem. Also, it may be that some other nearby piece, not this one, is the problem the other piece may be overlapping with yours. If you are using a vanilla kit piece, what happens if you build a small test room out of these pieces and go walk there in game? It is possible, but I highly doubt the kit piece is broken by itself.
