This seems to occur if you do a lot of upgrading in a house at once. Trophies will disappear from the stands, but will still be devoid of a placement option as though something is there, and Mannequins will sink into the stands and display as bare even if they have armor on them.

The bug is easily rectified by exiting the house and re-entering.

Thank you.

The mannequins won't display the Ancient Nord Helmet even though it's showing as equipped in the inventory screen. Which is too bad, I think the helmet is its best feature of the armor.