It has happened twice now. I went to my museum today to move all my promos and some of them where gone promos and all. Nobody but me and my alts have any permissions and I haven't touched the frames in over a month. Did I lose my promos? If so I lost a full set of 60k Member stuff, and a full set Boss Gear minus a Marlix Helm. What happened?
Nothing anyone but staff can tell. The only experience I've had with disappearing items is sugarcane. I have some planted around my house and every once in a while one up to three plants will completely disappear. But I've never had any issues with item frames. My storage uses item frames and I have a wall of item frames which contain items which varies people have sent me. This has been in place for months now (at least since March this year) and no accidents so far... So: while in game try using /staff and contact one of them to see if they can figure this out.
