This is a side effect of the way the art was created -- as it was originally created as zone objects, there was no underside (same with various other furnitures). Nowdays when they make objects they're pretty good at making backs and undersides so they can be converted into furniture (hm, do you think they've noticed me hanging around their offices going "ooh, pretty, I want"?

), but older stuff wasn't always made that way. You could try /bugging it, and I know there's a bunch of stuff like this (and bookcases) that's sitting in various artists' bug/to-do lists, but apparently it's pretty fiddly to fix and not a high priority. In the mean time it's just a choice between no house item at all, or house item missing an underside; all I can do is completely remove the no-underside items, but I suspect nobody would like that solution.
I think I saw some positions for artists advertised recently on the SOE job openings page though -- if you know any keen artists who also like house decorating, you could always encourage them to apply!
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