I'm currently on a Bedrock realm with friends, after a couple of logs in by my friends in further away chunks, a moving Hopper Minecarts that I have will disappear after I log onto the realm. 

I have a feeling its to do with the loaded/unloaded chunks. The pattern seems to be that if I'm the only one logging in and out, the problem doesn't occur as quit and load back in the same area. When I've not been playing for a few days and join the hopper minecarts will disappear.