They certainly do and I've picked up two FWB Sports on the cheap due to this very fact.
However, when fettled they are still very fine guns.
I think they have solved the degradation issues (?) on modern seals so this shouldn't happen on those guns where seals have been replaced. I've also had the backing washer on a BSA Scorpion pistol also go to mush so it would seem that there are a few manufacturers have fallen into this trap of incorrect spec plastic use.
Apologies for thread hijack. I certainly hope capitalbee get this rear sight sorted.
Dave