My experience is with a late JD "B" tranny which is very similar to the model 50. One of the possibilities for jumping out of gear is the detent spring on that shift rail is weak or broken. Or the notch in the rail is damaged. Could be several other things, I'm sure, but this may be the most common.
Maybe the ball on the end of the shift lever has broken off. I believe it would be possible to position it in any of the 3-5-6 notches, but would not actually move any shift forks to move a gear. Trying to move to the 1-2-4-R side involves the shift lever end moving the low/high range fork. If it doesn't move it won't allow the shift lever to lock into any of the "lower range" positions. These are just my best guesses as to what may be wrong. The problem I was working on with the B, was rusted shift forks/rails, and rusted sliding gears and shafts. Couldn't shift anything. Had to disassemble the tranny to do this, plus replace some parts and all bearings. Not fun, but challenging. lol
A side note for those who have one jumping out of gear: for safety purposes, it might be better to put a wedge into the gear quadrant slot to keep anyone from using that gear. It can be dangerous if they jump out of gear, to the transmission, as well as the operator, until it can be repaired completely.
It will be very beneficial to get a Service Repair Manual for the 50 before starting the repair, as you don't at this point know how far you might have to tear it down. The B is somewhat different than a 50 in the dash and steering rod support, but you need to start there to remove the shift quadrant, shift lever, etc. If the ball end of the shift lever is broken off, be sure you find all the pieces. If it is broken, it was probably caused by the gear trying to jump with only the lever holding it. The shift lever pretty much just moves stuff around, but not the intended locking device. Replacing detent springs or buttons requires removal of the clutch, first reduction gear cover, flywheel, and 4-6 shifter cover, to name a few. A 50 has some additional parts due to the wonderful live PTO that a B only wishes it had.
Keep us posted on the progress!
tommyhawk