Deleted Items on Game Startup

Hello. I just started using Hopper Minecarts to help collect items in my game, and for a while my single hopper minecart would be fine when logging in. Over time, I increased the size and had 4 hopper minecarts. They work fine, but when I save the game and shut it off, then turn it back on and log back into my game, 1-3 of those minecarts are just deleted. I keep my game to invite only, and I know nobody's been in my game to delete them. I'm running out of resources to keep making these every time I want to play.

Thank you for taking time to read this. Hope it helps.