A bug that's been on the console for a while is where if Minecraft is turned off in an abnormal way, the seed itself will be deleted from seeming both the Xbox and the Cloud. I've had this happen multiple times for my personal worlds, and every time I had to reset to the beginning because of it. I think it may have to do with the world saving when it closes. For this recent occurrence, it happened due to a blackout, so it may be tied to unplugging the entire Xbox.