Dual 4k monitors - only one works at a time though both are discoverable

I recently purchased a couple 4k monitors for my Thinkpad P51s setup with a dock. I previously used two 1080p monitors with no issues via DisplayPort => HDMI connectors from the dock => monitors.

Now with DisplayPort => DisplayPort connections to the monitors, both are discoverable (and light up during boot), but only one is used by Ubuntu once Ubuntu boots up to the login screen.

They are both discoverable in Ubuntu 20.04, and I can make either primary, but even if I have the second one configured as secondary, only the primary displays.

The only thing that has physically changed is the dock (I have the exact same model, but I left the other one at a different workstation), and the introduction of 4k monitors at 60Hz instead of 1080p monitors at lesser frequency (~30Hz).

How should I go about troubleshooting this?

Some specs:

I was using X.Org X server -- Noveau display driver from xserver-xorg-video-nouveau (open source), but switched to Using NVIDIA driver metapackage from nvidia-driver-470 (proprietary, tested).

sudo lshw -C video
  *-display                 
       description: VGA compatible controller
       product: HD Graphics 620
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 02
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:135 memory:eb000000-ebffffff memory:80000000-8fffffff ioport:e000(size=64) memory:c0000-dffff
  *-display
       description: 3D controller
       product: GM108GLM [Quadro M520 Mobile]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:02:00.0
       version: a2
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list
       configuration: driver=nouveau latency=0
       resources: irq:136 memory:ec000000-ecffffff memory:90000000-9fffffff memory:a0000000-a1ffffff ioport:d000(size=128)

xrandr
Screen 0: minimum 320 x 200, current 3840 x 2872, maximum 16384 x 16384
eDP-1 connected (normal left inverted right x axis y axis)
   1920x1080     60.01 +  60.01    59.97    59.96    59.93  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     60.02  
   1440x900      59.89  
   1400x900      59.96    59.88  
   1280x960      60.00  
   1440x810      60.00    59.97  
   1368x768      59.88    59.85  
   1360x768      59.80    59.96  
   1280x800      59.99    59.97    59.81    59.91  
   1152x864      60.00  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   800x512       60.17  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   720x450       59.89  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   680x384       59.80    59.96  
   640x400       59.88    59.98  
   576x432       60.06  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-2-1 connected primary 3840x2160+0+712 (normal left inverted right x axis y axis) 597mm x 336mm
   3840x2160     60.00*+  60.00    59.94    30.00    25.00    24.00    29.97    23.98  
   2560x1440     59.95  
   2048x1280     59.99  
   1920x1080     60.00    60.00    50.00    59.94    25.00    24.00    23.98  
   1600x1200     60.00  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1280x720      60.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
DP-2-2 connected (normal left inverted right x axis y axis)
   3840x2160     60.00 +  60.00    59.94    30.00    25.00    24.00    29.97    23.98  
   2560x1440     59.95  
   2048x1280     59.99  
   1920x1080     60.00    60.00    50.00    59.94    25.00    24.00    23.98  
   1600x1200     60.00  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1280x720      60.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
DP-2-3 disconnected (normal left inverted right x axis y axis)

• 4k via laptop or dock DP connection
• 4x via USB-C to DP cable

This is a possible solution according to a discussion on Ars. Their discussion is specific to the Thinkpad P51 which is the same model.