This issue seems to be unresolved.
Today i reinstalled OSMC with KODI 19.1 (August 2021 image) since the November update brought massive playback issues. While doing this i also wanted to reactivate my Ambilight build (WS2801 strip, no external video grabber). I configured hyperion.ng via the web interface and couldnt get the internal screen grabber to work properly. But the Hyperion Remote App worked like a charm.
One thing i did notice: Hyperion seems to be grabbing one of the desktop instances, but not the one where the video playback happens. One time i had some blue LEDs light up, and a Live Screen grab in the Web interface showed me an error screen with the frowning osmc smiley, but simultaneously console text on the right side of the screen with a black background. The text was a bunch of undervoltage error messages, and this screen is also displayed when i reboot/power down KODI and the main Desktop is closed.
Sorry i cant be any clearer, i am not versed well enough in the necessary jargon. I hope this helps with solving the issue, because all in all hyperion.ng seems a big step up in user-friendliness from the old system. Especially the web-config is much appreciated.
One last thing - in the old hyperion config tool it was possible to define edge LEDS, is this still possible? The current config preview for my setup shows 4 LEDs overlapping in the corners.