First time that RMT bit me, I've had crumbling couplers from almost all the other brands.
We all know that American manufacturers use contract manufacturers in the orient.
I know that the CM's all have their internal go to companies for certain materials. Example: I was watching Mr Kim (Kim Bjong Ki, ops manager at Ajin) repair a RoW tender with new decals. These were just magnificent. I asked where he got them. He said: downtown. I persisted and got the same answer with a look that said go away.
I tend to believe that there may be a die cast house that specialized in couplers. I've seen couplers from RoW and 3rd rail that have failed and the syndrome continues.
With that, I can only assume that the CM's over there continue to return to the same internal CM for couplers; sort of a roll of the dice.
Just my 2c.