I've seen this before where they did the "resign or get fired" BS and the person resigned, but fought it and they were forced to bring them back. The person retained everything and got back pay.
First it needs to be determined that the other sub isn't back because of a dispute. Seems odd someone would quit, but come back, so that alone sounds like something else is happening. We are an "at will" organization. We're not forced to fulfill a contract like the military or something.
If not and I was the new RCA I would file a grievance. Especially if there has been new assignments made since the last time the other RCA left.
Either way the union steward could find out what's going on if they contact the DR.