When you are in a Minecart, all custom button mapping will revert to the default settings. (When you get back out of the Minecart, all custom button mapping will return to whatever you have customized it to.)

Long, unnecessary explanation:
If you go into settings and you change the button mapping - say, you've assigned the Crouch action to the RB button - then you get into a Minecart and you try to press RB to get out of the Minecart, it will not let you out. You'll have to press whatever the default button is for Crouch before it will let you out of the Minecart. This is also true with all of the other buttons like Place and Break and Directions and Item Selection, etc. When you're in a Minecart, it doesn't care what you want your buttons to do. Only the default button/action assignments work. It's very annoying and confusing. This problem has existed since the first release of Bedrock on the X1.