First, using the current cabling (Dell mDP to DP cable and retail DP to DP cable), swap the monitors and their MST settings and retest.

Second, test using only the Dell supplied mDP to DP cables and retest.

XPS 8700 AMD Radeon R9 270 DP out --> P2415Q DP to mDP cable --> #1 P2415Q mDP in MST Primary
#1 P2415Q DP out --> P2415Q DP to mDP cable --> #2 P2415Q mDP in MST Secondary

Swapping the monitors produced same results (flickering in MST mode).

Using two Dell mDP->DP cables produced the same behavior as a Dell cable and a retail DP to DP.

Some datapoints;

In the Radeon tool I saw that the primary monitor was set to 10 bytes per pixel and secondary to 8. I changed so both were 8.

Windows consistently detects the primary MST monitor as Generic PnP and the secondary monitor as P2415Q.

Is there another system to test these on to see if the issue follows the monitors?

I dont have another PC thats capable of driving 2x4K

I do have an iMac 5K Late 2015 model with a Radeon R9 M395X 4GB, that is said to be capable of driving 2 4096x2160 displays.

  Im happy to try tis though I understand that Dell doesnt provide full support or extensive testing with  Apple hardware

With the iMac connected with a thunderbolt cable to the monitor thats configured as primary MST I see dual 4K @30Hz, no flickering but with each Dell monitor showing the same mirrored content. That's with the other monitor configured as MST secondary. I see this regardless whether I use a Dell mDP<-->DP cable or the retail DP<-->DP cable. When I disable MST on the secondary monitor nothing changes.