I would not want to speculate as to why the leather is sticking to
the valve plates.  I had a similar problem on a Leonard player once.
I took all the valve plates off and soaked them in lacquer thinner,
then I lightly polished them with a soft cloth.  I used smooth leather
from Tandy Leather that worked very well.

I painted them with automotive paint because of rust problems and not
having the ability to have them replated.  I may have used talcum
powder on the leather.  That was twenty five years ago and this piano
still works very well.

Don Teach