From Dolby on BluRay Authoring:
Dolby Vision Graphics Priority mode
"When graphics are the focus onscreen, it is desirable to program the UHD Blu-ray Disc Player to optimize the picture accordingly. Properties are now available to Blu-ray Disc Java (BD-J) programmers to explicitly switch between these modes.
Note: In HDMV mode, graphics priority is the default setting.
To maintain a consistent graphics and brightness level, a disc author can invoke Graphics Priority mode when graphics are present. The video is still rendered well, although certain advanced picture rendering technologies are disabled. Therefore, it is important to keep playback as the default Video Priority mode whenever graphics are not present, as well as for certain kinds of graphics, such as subtitles."
Please check page 16.
This was the key to understand and cross reference to the code.
The AMLogic code is setting this on each frame to graphics priority - (a bug in my opinion), quite possibly someone misunderstood it’s meaning/implication - as you say with all the talk on LLDV - the change was made in the code in 2020.