Same issue here.
HP Spectre x360 13t with 4K display
Intel Core i7-8550u with Intel UHD Graphics 620 driver version 23.20.16.5044
Only the internal display dims after reboot. An external monitor attached via USB-C to HDMI hub doesn't change.
Under advanced power settings - Display there is only the settings for Turn off Display after - On Battery and Plugged in. There is no option for adaptive brightness or any other options.