MUAHAHAH I have this SAME problem on my Gottlieb Pinball Pool. It is so frustrating to find the issue.
First when I was playing, the left pop was doing this. The ring would stick down, and turning the game off, it stayed down. If the pf was raised or you pulled the ring, it would come up, but it seemed/felt like it was magnetized plunger to coil stop.

I ended up ordering all new parts for that pop and in the mean time, swapped parts between the two and then when I put them back, the right pop started to have the issue. I have rebuilt the left and that seems to work 99%. The right is now acting magnetized and I just now, pulled it apart, hit the parts with a hammer, left in some water for a bit and hit with a hammer again. I'll rebuild when try and try again. Otherwise, i'll need another order....

I had to order new coil stop and plunger. Hitting with a hammer, the small red dema/mag didn’t work. I’ll update when the parts come. I’ve never had or seen this issue before so it took me a while to figure out

Update:

Can confirm the new parts fixed the magnetizing/sticking. I also put new springs in.

yeah, it really makes you feel like it is binding...but man it was just like....a WTF. Once rebuilt...bam, super smooth. If you play with it, you'll see and feel the magnetized parts but imo, just worth rebuilding and not even worrying.