Joining a world I've played on before and having to start over then next time I load it up I have my old data back

My girlfriend and I have a world we've been playing on since village and pillage. We recently made the world into a realm and downloaded it to the Xbox so we can play it more often and easily. When I went to join it made me do a whole new start. No big deal. I went to where we were grabbed stuff out of the chests of my old save data and went about playing the world. Got off and got on the next day and was using the same save data. Got off and on two days after the download and suddenly my previous data is back causing me to lose all the items I had with me on the other save data. I'm disappointed because some of the items were ones I've had since the beginning of the world and had some memories and stories with them. I know I can just get the items again no big deal but this issue has happened before