We built a small "shrine" around the world spawn point so that new visitors to our multiplayer realm arrive in a friendly and attractive location. Over time, blocks of the wall, ceiling, and floor of this shrine are "destroyed" as if they've been mined--as in, they appear as spinning items before completely despawning. This includes quartz blocks from the wall and floor, quartz slabs and stairs from the ceiling, and panes of glass.

We have several written books on lecterns in the shrine that so far have not despawned or been destroyed (thankfully). One other note: I've observed that the longer I go without checking on the shrine building, the more blocks will disappear. If I check it every 2-3 days (real time, not Minecraft time), there will generally be 4-6 missing blocks. However, I decided to test it by not going near the shrine for 2-3 weeks, and when I visited today, most of the wall, ceiling, and floor were gone (picture attached).

I'm guessing this has something to do with the fact that we've built an enclosure around the world spawn; maybe the game doesn't like for players who spawn in fresh to potentially be trapped somewhere. Seems like a clumsy way to deal with it though, and after extensive googling, I can't find any other instances of people mentioning this issue. Anyway, I can't imagine this is intended behavior (there is a door on the building after all). It's definitely not related to new players spawning into the realm for the first time, since we haven't had new visitors in quite a while. Should note also that this was also happening prior to the recent 1.16.100 update.