As far as I'm concerned is a bag of spanners!
Been fighting my Mk3S with MMU2 ever since I upgraded the FW. Downgraded back to 3.12 and the filament loaded immediately, no issues. Fed all the way to the extruder. Noted that 3.13 didn't feed all the way to the extruder. Even chopped a bit off the PTFRE tube but that never helped.
No idea what was done in the supposed upgrade from 3.12 to 3.13 but it was obviously not tested very well. Won't be trusting any more FW upgrades in the future!