P2415Q, Surface Book 2, Surface Dock, 6-bit color FIXED

By choosing "On" which is really Primary and Secondary, you are now using the P2415Q at 30Hz which is possibly why it could then do 8-bit color.

 

The P2415Q Menu- Display- MST has three options;

Off = 4k2k 3840x2160 60Hz, MST Disabled
Primary = 4k2k 3840x2160 30Hz, MST DP out Enabled
Secondary = 4k2k 3840x2160 30Hz, MST DP out Enabled, Supports DP 1.1 for older DP 1.1 video cards

I am not sure that backing down to 30Hz would be a fix, more of a workaround. But most users would not want to stay at 30Hz.

This is a really going point Chris! its funny because i can certainly notice the difference between 30hz and 60hz when it comes to mouse movements! I guess the problem then still remains with the intel drivers as this was never a problem before some of the recent updates. with your tips i feel i can better report this issue to microsoft. i do also think it may have something to do with the dell monitor. i have read reports of other dell monitors exhibiting the same issues. certainly further investigation is required. i hope dell can look into this as well. it's not my job to fix their products, or intel's, or microsoft's. what i can tell you in that when i bought my surface book 2 and dock and used it with the dell 4k monitor, it worked perfect out of the box. 4k, 60hz, 8-bit colour. now a year or two later and many microsoft updates and intel driver updates....its not working :(