You will removed by player riding before reopening the world if you riding
How to stop removing by reopening world
1. You need exit the cart
2. You ready to reopen it!
This things is the bug
MCPE-8235
Minecart has removed by player riding before reopening the world
Resolved
Duplicate
1
If you have not already, I recommend performing a search before submitting a report.
This issue was reported previously as MCPE-7957, comment your experience there!