Precision 3571, WD22T4, dual 4k@60 not working.

Hoping someone here might have some ideas why this isn't working.  As far as I can see from spec sheets, all this should work just fine - I should even be able to run a third 4k@60Hz screen.

• Precision 3571
• 2x S3221QS connected via HDMI (monitor end) to WD22TB4 DP
• Cables are 8k@60Hz capable, HDMI 2.1 to DP 1.4.
• The internal display is disabled/disconnected (not that it should matter, but to remove a variable)

I cannot run both external screens at 60Hz.

Either screen can run at 60Hz, but only if the other is configured to run at 30Hz. So it's not a monitor, cable or port problem.

I can connect either of the two screens to the HDMI port on the laptop itself and run 2x 4k@60 (one via WD22TB4 & one via internal HDMI), but this is obviously a sub-optimal setup.

Above is my home setup. At the office I use the following:

1x 4k @ 60Hz connected through to HDMI on portable UGreen USB docking station connected to laptop TB port

1x 4k @ 60Hz connected to the laptop's HDMI port.

Internal screen 1080 @ 60Hz.

I also tried connecting the monitors using the two TB ports on the docking station and the USBC->DP ports that are normally connected to my Mac Mini and that works fine (2x 4k@60Hz).  So it seems to be something specific to the 2x DP ports on the WD22T.