I have an issue with 6bit colour on 1 of the 2 monitors.
More often problem appears in the monitor which connect to the doc station via DP. For me helps (not always) to swap cables (DP and HDMI) but after my system will be rebooted the issue always appears again. And it is very very tiring to constantly swap cables.
My system is:
1. Dell XPS 13 9360-5563 (UHD Graphics 620)
2. Dell Thunderbolt Dock (TB16)
3. 2 Dell U2715H