I was having this problem as well. I kept having to go into setting --> vehicle --> rearview camera and turn the active + fixed lines back on.
Another fellow in the MFT thread told me he noticed that the option was getting deleted when he put the car into reverse before MFT had fully booted up before putting the car into reverse. I have been making a point of not reversing before the screen is fully booted and I have not had that issue since.
Ideally, the software should not work this way, but . . .