This is interesting. Good detective work there.
I think this is the the ZF 5HP19 transmission. If so, here's a page out of the manual that shows the parts numbers for others to reference.
I'm a little unclear on what you're telling us. I think what you're telling us is that the piston wears out the bore, which also wears the piston. The fluid is eventually able to bypass the piston. This causes the transmission failure. The solution to this is to replace the piston. Is that the essence of this?