XPS 9300 Developer Edition + WD19TB: Dual monitors detected, no signal

Unboxed my new XPS 9300 (Ubuntu 18.04, i7, 32GB) and WD19TB dock today. I purchased this specifically for its ability to drive multiple external monitors.

After going through setup, I connected the dock and 2 x 4K monitors via DisplayPort. Only one monitor displayed video. The other is detected, appears in the “Displays” settings area, but I can’t activate it.

Occasionally, it will show as active, I can move the pointer through it, but there is no video output. Other times, both external monitors will show as detected and active, but I get no video to either of them...

• Tried updating all Ubuntu packages and kernel, no change.
• Tried multiple reboots, cycling Thunderbolt connection, no change
• Verified both DisplayPort outputs are working - they both drive a monitor without issue when connected individually.
• Tried connecting Thunderbolt to other side of machine, no change (other forum posts indicate only the right-side port offers full capabilities)
• Tried updating the bios to 1.0.10, no change 
• Tried Using DP to HDMI adapters in various permutations, no change
• Tried running monitors at lower resolutions, no change
• Tried attaching 1080p max native monitors instead of 4K, no change 
• Tried reloading the factory image fresh, no change
• Tried booting into Pop_OS! Intel 20.04 liveUSB, no change

Not sure how to troubleshoot from here. Any suggestions welcome! I’m very disappointed by external video problems right out the gate on a $2500 system that’s supposed to be Ubuntu-certified, and is running a Dell-supplied distro build.

Thanks,
Dave