If we get cast iron proof that the video priority is allowing the RPU to be passed and processed on the display end (tv-led), then the below is my take on what was going on with fake tv-led at least for this device:
graphics priority is meant to be for HDVM and for other Java based graphics etc on BluRay players, Can think of it like computer graphics - the device is controlling the drawing and making graphics which it wants to control on the 2 OSD planes, and not have those images pulled around by the image processing (rpu processing etc.) on the video plane.
At the same time we are asking the device to be tv-led so tunnel the data with rpu data.
It is then doing the best it can to process the rpu on the video plane itself, then blend with the two OSD planes and sending out - it does not want the rpu processing on the tv / display side distorting the graphics so removes/nulls out that data so nothing is processed that side.
if my take is right fake tv-led was just graphics priority mode all along doing it’s best.