I found them to be buggy too. I changed what I originally put on one and it changed back. I put the Masque of Clavicus Vile on one and it replaced it with a helmet I removed. It respawned a helmet in other words. The Masque was still on the mannequin inventory and I took it back. I was able to get free Nordic Carved armor. I put dumner clothes on one, left and then removed the clothes and put the armor on it. I left again and returned to find the dumner clothes respawned and were on the mannequin. The armor still showed so i took it and left again. I came back and found partial armor on the mannequin. So due to the glitch, I got a few pieces of free armor to sell. I leave them as they are now. It did not respawn the Masque, but I better check next time I am there.