When I was made a building and logged off, players of the realms asked when it was going to be done. I came back online and saw that half of the building went back about an hour. I re-built that side and tested to make sure it was working, and logged off. People then told me it wasn't working again. Sure enough, it went back to the old way.
It's especially strange because I started working on that side first, so it wasn't the whole world that went back to a previous save. (And I am positive no players would be able to play a prank on me.)
The screenshots are examples of how the shop looked before I logged off, and how it looked after with one side going back to a previous time.
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migrated
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I don't know if you've heard about them, but it could be a corrupted chunk. They can't save/overwrite, so revert to previous known states. I think you might need MC Edit to regenerate the chunk, as far as I know how that works. If you can download the realm save, edit it (I don't know any of the regeneration details) and reupload it, it should work. Hopefully.
kumasasa
Duplicate of MC-2548
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I've had this same problem just last night. My friends and I are building a mini-game server out of redstone. Anyways, we got some pretty good work done. Next morning my friend calls me up and tells me none of it saved! I was able to revert to a backup to save it, but then it happened twice more...and it's not corrupted chunks. I had corrupted chunks in a singleplayer survival world and fixed them with MCEdit.