Quoted from MarkG:

This sounds like the problem addressed here:
https://pinside.com/pinball/forum/topic/fixing-the-%e2%80%9cextra-ball-after-game-over%e2%80%9d-in-a-gottlieb-single-player-em

I've had some luck repairing worn stepper rivets with silver solder. It's harder than standard solder and may hold up to home use. Clean what's left of the old rivet, maybe add a little flux, then solder on a healthy blob of silver solder. You may need to file it down to the correct shape and smooth or polish it some too. Alternatively PBR may be able to replace the rivet, or the whole disk.
/Mark