charliemora2There are two possible reasons, the pins at least A&B 2,3,10,11 are not connected/present or the absence of the protocol in the Aerocooler firmware.
I came here looking for the same answer/fix. I own and used the ROG Phone 2, 3 & now the 6 Pro stricktly as gaming devices and live stream. I had No issues with either of the predecessor models, but the upgrade/downgrade of the Aero Active Cooler 6 is making me regret my purchase. Asus please release a firmware update to re-enable the video out.