Item frames and Items in items frames randomly disappearing

The item frames in my spawn city are randomly disappearing forever. Sometimes the frame itself stays but the item disappears. Seems to happen randomly when no one is on the server. I have a lot of item frames decorating my spawn city so they it is very noticeable when they disappear.

This has actually been happening for the last few weeks and I thought people were just destroying the frames but they can't (as far as I know) since everyone is in adventure mode with a mining fatigue effect on them while in spawn city.