Setup Custom Resolution on LG C2 with AMD

i upgraded my gpu to an amd 6900xt and try to set ever think up, but i have problems getting 12 bit color to run. i run true all guides but cant really find a solution for it. when i am on normal 4k screen mode, i see:

• 8 bpc
• 10 bpc
• 12 bpc

in the amd control panel. i can change between all 3, but when i choose the 12 bpc and press the green button many times, it shows me that just 8 bit is active. 10 bpc is working as expected here.

it gets even more messy when i try to set ip my 21:9 mode. when i create the new resolution in the amd software with 3840X1600@120fps, i just see

• 8 bpc
• 10 bpc

and can only activate the 8 bit option. the main guide here says to set up a resolution to 3840X1620, but when i do that, the picture gets stretched to full screen.