I am playing on a server in snapshot version 21w07a, and sometimes after reloging or entering a nether portal chunks will get a setback. it looks like sometimes chunks do not save properly after unloading them. and i am pretty sure its not server sided because i have changed the same server to version 1.16.5 and no issues were found there. im not 100% sure what causes this bug because it looks like it is a bit random but most of the time it happens when i am going thruw nether portals alot.