Hey, I just tested the official HDMI adapter on standard non rooted android 8.1 and it works fine to transmit image.
I do however get a quirk that only displays the correct orientation while on lock screen mode. Once logging in I get a 90ยบ rotation of the mirrored image, wasting much space on my screen and also requires turning my head or monitor.
the "Force rotate" rotates the content of the embedded screen and mirror that to be displayed on the monitor, but still only in the middle of the big monitor.
While changing states in "force rotate" as well as just before going to lockscreen and screen turns off due to inactivity, the monitor flashes the full monitor correctly for less than a second.
[edit]just to clarify: If I use the integrated force rotate, the display on the device will switch to portrait mode, and the image displayed on the external monitor will still only be shown on the same part of the monitor and then reflect the portrait mode on the gemini (I dont remember if this was even up-side down)[/edit]