Most fluxes available today are not corosive ( EPAgets into everything ) Even so in the electronics industry we use nylon hardware all the time and have no issues. I doubt the solder or flux had anything to do with it. My guess is that they were heat damaged. Either the brass had some residual heat still from the soldering or the clevises were a tight fit on the coupler and enough friction was generated to create enough heat to damage the plastic. You could just be right and they were old but Ihave never run into that myself.