It is not white rot or you would see dramatic white fungus at the base of all onions, and they go yellow very fast. Sounds like mildew, or possibly onion fly. Are there any maggots to be found in the bulbs/ Or more likely, are any green leaves looking mouldy? In either case the onions are fine to eat.
I cover it in This Month. If everyone is affected I suggest you all agree not to grow the overwintering Japanese onions which harbour mildew. White Lisbon are ok.