Ugh, You still can't get into custom mode?
What shard are you on?
Maybe you just need a fresh set of eyes to look and see if there's any other deeded items you may be overlooking.
It probably really is a bug, nothing to do with you overlooking something but it couldn't hurt.
If you let me know shard, facet, coords etc and if its public I'm willing to come look. I'm not really customize guru - that would def be Vlad - but I know my deco items. haha
On top of that I'd suggest submitting a bug report:
Feedback | Mythic Entertainment | Ultima Online
Possibly PMing or emailing Mesanna. I know she's crazy swamped but if you submit the report, have tried paging gms multiple times, and have had other people come look to see if you missed anything.. Messaging her might be the best way to go.
I know there's always been a bug with items falling through the floor and getting stuck under your house, I wonder if that could happen with a deed or deeded item.