I am not a mechanic so I can not directly answer your question but I did a little inquiring via internet and there are many possible answers to your question. Search for "harley shifter shaft loose" or similar and you might find it.
riding down the expressway, took my foot off the highway peg & set it on the foot board. something felt weird. look down, there's my shift linkage (with shaft) just laying there. how i didn't lose it somewhere on the road, I'll never know. piss poor design. I carry some tools with me, so I was able to put it back together. weird thing was there's a 1/4 inch screw that goes through lever and rides in a groove in the shaft. the screw was still in there, tight as hell. the shaft didn't look particularly worn, but somehow it squeezed out past that screw. to think that's all that holds you shift linkage on is just stupid. rode from Tenn. to Ill. watching that damn shifter more than the road. anyway, put a new screw in, been okay since. it's just a regular hardware store screw. I think it only has to wear a tiny bit to be a problem. fyi, thought of using a hardened screw, but that might wear the groove in the shaft. easier to replace the screw every couple years. seems like a pretty sloppy design for a pretty important feature.
Had that exact same thing happen to me on a two week old Road King. If the screw gets loose on the pinched linkage opens slightly it will allow room for the screw to slip over the grove and the linkage fall off.
Use a good amount of Loctite on that screw!!!