I created a build over the course of a few hours and then logged off of my Minecraft realm which I have had for about six months now. When I logged back in about thirty minutes ago, it had reverted to before I logged in previously. The build was gone and my inventory was reverted as well.
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also had the EXACT SAME PROBLEM. I just found a mangrove forest and frogs and was collecting stuff when I came back next day i was back at my house inventory reset as well as world generation and mobs that I had killed only saved thing was broken blocks

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I started having this EXACT same issue just a few days ago. About 10 hours of work was erased, and having checked it just now even me moving my character around to verify this was reverted to an older "save" or whatever they use for Realms.