There is supposed to be a white plastic bearing (# C-342) which snaps into those grooves. The parts catalog calls it a guide ring.

https://www.marcospecialties.com/pinball-parts/C-342

To expand on Ken's post that bearing keeps the plunger centered in the sleeve.

Not surprising. They're nylon/plastic and over time and age and heat and whatever they give up the ghost. People take the plunger out and lose them. They're one of those things that pops off and rolls under something and you can't find it so you order new ones. Three months later it magicall reappeasr and you have a spare. Then you need it again sometime and you can't remember where you put it.