Could those lost trophies be because of a bug with client/server side flags (properties) of items? I know there is a flag meant for all wall IDs, that decides whether a wall should allow hangables or not, and maybe this could mess something up. It could be a case of a mismatch between server and client-side (e.g client allows you to hang tapestry, but then server does not).
It could also be based on outside factors -- like maybe some squaremeters have double walls stacked on top of each other in the map editor; one with the hangable flag checked and the other wall with it unchecked. In this scenario it could be possible that you are able to hang your tapestry on the wall at the bottom of the stacked "double-wall" because it is a 'hangable' wall, but then in map editor another wall is placed on top of it which means you can never see the tapestry, and thus can't reach it to take it down.
If Iryont has exact locations of a bugged wall spot these two ideas should be fairly easy to test for.