With the car not running/ switched off..If you select low gear, then remove the cables and try to shift it back into neutral by hand at the trans does it slide out of first easily or require a bit of initial effort... if it slides out easily its likely a detent spring is missing or collapsed. You also mention it 'feels' like its going into gear fully... Jason Ferraro had an issue with a selector sleeve bush that had moved in his Ricardo causing problems with 4th? gear IIRC, I had a quick look at my Ricardo data & Drawings, but I cannot say for certain that this could apply in your case.
Can a Synchro go bad, begets the question, does the gearbox protest/crunch when shifting from 2nd to 1st, if not your synchro is OK, but the dog ramps of low gear 'could' have taper wear in your case if its been slipping out of low gear on deceleration like this on many occasions, another possible cause could be failed or missing thrust washer on low gear.