Graphic Card: RX480
Radeon Driver: 19.8.1 (same with stable 19.5.1+)
Monitor: BenQ EW3270u
Connection cable: DisplayPort
OS: Windows 10 1903 (was same with 1809)
In RGB444 / 4K / 60hz mode, I can't enable 12-bit color depth.
The AMD control panel can see the menu for 12 bit option, but selecting 12 bit blinks the screen (as if it tried to apply it), but comes up reverted to 10 bit.
Note that behavior is exactly the same as it was for 10-bit in this older thread, but the 10-bit issue has been fixed in 19.5.1:
Windows 10 RGB444 4K 60hz 10bits mode issue
Note also that the screen is generally capable of 12 bit (it can be enabled successfully at 30Hz), and BenQ advertises this screen as DP1.4 (which should provide sufficient bandwidth).