Encountered this today with F2 loaded to nozzle (Filament read "?" due to power cycle since it was loaded). Mk3S FW 3.9.0-RC w/ 1.0.6.
Went to first-layer calibrate a new sheet, accidentally selected F1. Printer printed intro line and screen then changed to "?>F1". Extruder unloaded, and the MMU moved the selector to slot 1 despite the fact there was still filament in the FINDA (light was out, FINDA has been adjusted and has never given me a false runout during prints)
Of course it jammed and I had to partially unmount the selector motor to unbind things.