this issue happened when i upgraded my server. Whenever I went through a nether portal and came back all the chunks were set back to what they were before. this applies to both nether and overworld.

as an example, you could break a chest in 21w07a that you placed back in 21w06a and when you went through a nether portal to the nether then back to the overworld the chest is back but the entities are still there.

i couldn't replicate this in survival.

edit: when i swaped to a backup on the previous snapshot the issue still happened but rarely.