I had 2 simular rings , both equiped , one with materia attached to it and one empty.
When i melded materia to the emtpy ring succesful it didn't add the materia to the empty ring.
Looks like the game decides that when you have 2 equal rings , it takes the ring that is first on the list(on top) , but it did not add any materia to either one of the rings , so the materia was lost.