I have noticed that when I search for J flights on two leg trips with Avios something similar happens. It offers a flight with one leg in J and the other in Y. If you don’t read all the information carefully you might book it expecting a complete J journey and be unpleasantly surprised!

Incidentally, you also need to be careful with Royal Jordanian Flights that connect in AMM onto a BA flight to London. The BA leg is not proper J. They use a narrow bodied aeroplane so it is the normal economy cabin that they pretend is J by calling it Club Europe.