Android TV 9 screen resolution limited

I installed last-built android TV 9.0 firmware (20200827) on my rock pi 4B. I connected it to a UHD 4K TV. Everything went well. However, the maximum screen resolution shown in the list is only 1920x1080. I’ve deleted resolution_white.xml in system/usr/share, after reboot, I still can’t get 3840*2160 available. Please give me some advice to solve this problem. Thanks!

The HDMI resolution is 4K(check your monitor input report), and the Android UI is 1080P but when you play 4K video, the output video resolution is 4K.

Thanks for your reply! I found it might be the cable problem. After I remove the HDMI extension cable, the 4K resolution is finally avaliable!