Can you check your PDK document to see which layers are needed to define a diode you're using in schematic? You can find this information on a table probably named device truth table or similar. Because it sounds like you need to add DIODE layer to define separate diodes, otherwise it would probably check for parasitic diode (DIODENWX sounds like a parasitic diode but I'm not sure as I've never used AMS018) rules which may require something else to be around.