I've been playing on a vanilla snapshot server with friends, when I encountered a bug where chunks would fail to update / save after blocks were placed or removed. This is only effecting a select few chunks.
In the screenshots you can see me place two grimstone blocks down. After this, I leave the server and rejoin, the pillar is gone and I am missing my two grimstone blocks.
This effectively makes this area impossible to build in, as anything you place is lost once you leave. I'm not sure why chunks are straight up not updating or saving, I did nothing out of the ordinary.
I have tried to restart the server to fix the issue, but it persists even after that.
EDIT: I have tested this further, and after I killed myself in game from fall damage, the issue no longer occurs. I have no idea what caused it to trigger in the first place, but it definitely seems like a pretty big issue as you can easily obtain infinite resources with it if you figure out how to purposefully trigger it.
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I experienced this bug a couple of times in a 21w11a server.
I also noticed that tile entities such as hoppers or furnaces were not updating, staying frozen in time while block interactions like placing water and having it flow would work just fine.
Leaving the server and joining back caused the chunks to "wake up" but all data on those chunks rolled back and progress didn't save.

Relates to MC-63
BlaCoiso, confirm. This problem often appears on the vanilla server 21w11a.
So the same happens to me, now after regenerating the chunks, they stay (at least until now).
I play single player. Also a whole village dissapeared.
Wait, if a village disappeared too, it must affect world gen in some way.
How 1.16.5 is affected?
We have been running Snapshots as a Server and can confirm this happens in parallel to furnaces, hoppers, chest, sticky pistons, etc breaking.
(Would venture to say this has been present in the first few snapshots and up until now has not been patched)
Manually running /save-all helps with fixing broken chunks (Regarding not saving) but furances, hoppers, chest, stickypistons, etc.. is less deterministic (Usually relogging, leaving the chunk, coming back sometimes fixes it)
Still Effects 21w17a
My experience with this is that chunks are not reliably saved when going through a nether portal. Frequently items that were placed or blocks that were broken immediately going through the nether portal are reset when you return.
affects 21w14a
Hmm, sounds similar to MC-216570. There may be something to this