Many of the features of the S905X4-J are not working on the -ng branch because support for this chipset was never really developed for the -ng branch. Basically, the -ng branch is considered End-of-Life and I think the CoreELEC team would like to see it discontinued.
The discovery that FEL works on the S905X-J overturns the dominant theory that FEL support is in hardware. For the longest time, the assumption was that FEL support was either enabled or disabled on the SOC and therefore nothing could be done about it. The S922X-J supported FEL and nothing else did. The discovery of this dovi.ko module and the ability to play FEL on kernel 4.9 proves that FEL support is in software not hardware. Therefore, there are two possibilities either Dolby disabled FEL support in later dovi.ko modules or there is a gigantic bug in the amlogic code similar to the one that disabled TV-led Dolby Vision until recently.
I see three realistic paths forwards. First and most likely, the CoreELEC team could work on bringing support for the S905X-J to the -ng branch. Second, if support for FEL is disabled in dovi.ko, someone could create a modified version that reenables support for FEL. Third, if it’s an Amlogic bug, the CoreELEC team could track down the bug and fix it enabling FEL on a wide range of devices.
In short, it’s time for some patience until the CoreELEC team can figure out the best path forwards.