I've been having the same issue as well.
Flying KMSP-KDFW - roughly over TUL and my engines turned off, fuel cutoff levers moved to OFF. Plenty of fuel, none in Center tanks. I did not have wing anti-ice or fuel heat on. I thought this could be it.
Continued to fly KDFW-KTUL - Passing FL190, with fuel heat and wing anti-ice on, fuel cutoff levers moved to off. Managed to recover and happened again after FL200.
I have nothing connected to the mixture or the fuel shutoff levers - so not sure what it could be.