is there a reason why the supout.rif is deleted after reboot of the device?

Because it most probably resides on "RAMFS". That's gone after reboot. You need to move it to the "flash" folder if you like to keep it.

But it is shown in the "File list" and I as a user cannot see that there is a difference of my backup files and these .rif files. At least there should be something like an indicator that its just written in the RAM.