I recently got the Realms 1-month trial. After being unable to replace the world on the first day I waited a few days and then was able to replace it. However, upon going into the game, I had my friend's inventory. That same friend joined the game and I figured since I was initially the 2nd player to join the game in the original server, that my friend would have my inventory. This, however, was not the case. My friend had an old inventory of theirs and I had their current one. After deciding to just deal with the regular world, I went back, only for my friend to have their current inventory and me, having the same identical inventory to them. I lost all my items and everyone else in the server seems to have their stuff. I'm really disappointed because I had a lot of time poured into the items I gathered in that world. We don't have any copies of said world, either. 
It should be noted that I made the realm with a copy of my friend's world. It wasn't originally my world. This might be a possible reason, but it still shouldn't be happening.