That痴 a common problem with Farmal and a lot of other H pattern older shifters.
The fix is take the 4 bolts off that hold the shifter down to the gear box. Lift the shifter up with the plate, look into the top and you will see haw the sliders push the gearing your forks are worn a little and one side R-1 0r the other 2-3 has failed to move to the center neutral position. With your hand place both sliders into the center neutral position, replace the cover with the shifter in the neutral position and it should work fine until someone once again tries a speed shift, usually between R and 2, If it continues to happen you can either replace the shifter forks OR just build up the dimples at the fork tips with a little brazing rod. KennyV
Fcubs are a fun little tractor, I have one..