I have build several nether portals, lit them, and entered the nether only to return to a brand new portal in the overworld. Also have dug in various dimensions, left the dimension and returned only to find the most recent work reverted (for example: bringing rabbits thru the nether, dug a hole to hold them in, gone to the overworld to block of the area so they cannot simply jump away, and return to the rabbits suffocating in the blocks I had removed. One more example of blocks regenerating: while digging in a straight line with a fast enough pickaxe, I have found my path filled in occasionally on my return trip Additionally, chests are not saving either. If I go exploring and place my new found items in a chest and log too quickly, I return to a chest that is missing my hard sought after items. The reverse is true, however, I can take items out of a chest and log or go to another dimension quickly and return to the items in both my inventory (where they belong) and in the chests they were removed from.
If I was to guess, the saving of the chucks is not happening often enough or not when the chucks are being unloaded. I have encountered these issues on various servers and in single player worlds for several snapshots going back to just before deep slate being added.
Linked issues
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Oh, this will get linked to MC-216148
This does indeed appear to be a duplicate of MC-216148, which was fixed in 21w18a.

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I can confirm this is an issue in 21w14a, where chunks are not saved reliably when going through a nether portal.
For instance, I placed a shulker went through the nether portal. Moments later I came back through the portal and the shulker was not there. It was not in my inventory either and relogging did not help the issue.
There have been other instances of chunks not being saved when going through nether portals on this server, so it is not limited to shulkers.
Sadly, we will be stuck with this massive bug, until work resumes on the Caves & Cliffs update. Just make sure that it doesn't work its way into the general release!