U2722D MST breaks when switching between input sources

I have a Dell Lattitude 5420 connected to two U2722 displays via a dell docking station (WD19).

The primary display is connected to the docking station via displayPort. The second display is daisy chained to the first monitor via displayPort.

The primary display has MST enabled, and everything works just fine. My desktop is extended across both displays.

BUT….

I additionally have a desktop PC connected to *both* displays via HDMI.

I have occasional need to switch between the laptop (via displayPort) and the PC (via HDMI).

Switching from the laptop (dP) to the desktop (HDMI) works just fine. Both displays are working just fine via HDMI.

Switching back from the desktop (HDMI) to the laptop (dP) results in the displays being mirrored, behaving as if MST (multi-stream transport) is disabled.

This is undesirable and seems to be a firmware bug with these displays.

I am only able to resolve the issue by explicitly disabling MST on the primary display, and then re-enabling MST on the primary display. At that point, my desktop is correctly extended across both displays once more.

I note that the firmware of the two displays differs (As reported by Dell Display Manager)

The primary display has firmware M3T102

The secondary display has firmware M000

I see no option to upgrade the firmware within Dell Display Manager.