Last month I had a flight delay for my incoming QR flight so I missed my connecting QR flight and rebooked to MH operated flight. There was no dedicated J bus for me so all J passengers were just given priority to board the bus first followed by Y passengers, with a very full bus and long queue on the staircase boarding the plane. I recall someone mentioning having mixed J and Y passengers in a bus in an earlier post or somewhere else.

I think the dedicated J buses might only be reserved for QR operated flights, so the transit experience was really poor if you're a J passenger on a non-QR operated flight and have to use a bus gate (which happens quite often nowadays).