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.