This was pointed out in the CX forum about oneworld lounge access via Class of Travel (J or F). OP in that thread otherwise had Ruby status. From the oneworld website (emphasis added):

Lounge Access by Tier Status
Emerald: When travelling on any of the oneworld member airlines, any customer who has earned Emerald tier status, is welcome (with one guest travelling on a flight operated and marketed by a oneworld airline) at any oneworld airlines' pre-flight lounges, including First Class, Business Class and frequent flyer lounges regardless of the class they are travelling in.

Sapphire: When travelling on any oneworld member airline, any customer with Sapphire tier status is welcome (with one guest travelling on a flight operated and marketed by a oneworld airline) at any oneworld airlines' pre-flight Business Class and frequent flyer lounges regardless of class of travel.

Lounge Access by Class of Travel
Customers flying First or Business Class have access to the equivalent class of lounge regardless of their frequent flyer status. (First Class passengers may be accompanied by one guest travelling on a flight operated and marketed by a oneworld airline).

Anyone else notice? Was this always how it was? Because I could have sworn that holders of a J or F boarding pass would have been welcome at ANY oneworld lounge (not just their carrier's lounge), because it clearly states that Sapphire or Emerald Tier members are welcome at any of the J or F class lounges, respectively. The OP in the other thread had asked because OP's onward connection in J was much later on in the day and the lounge wouldn't have been open, while partner carrier's lounges would be open.