60Hz on 4K monitor lost after reboot with Thunderbolt dock G2

[joern1970]

My setup:

HP Elitebook x360 1040 G8 with HP Thunderbolt Dock G2

Monitors:

1. Dell U4320Q 4K with USB-C at Thunderbolt output of dock

2. Dell P3222QE 4K at Display Port of dock

3. Dell U2412M 1920x1200 (16:10) at Display port of Dock.

This used to work with all monitors at 60 Hz. And it should work according to the Dock specifications.

Now I noticed that the monitor at the thunderbolt output of the dock  (Monitor 1) is only getting 4K 30Hz after reboot. 

If I reboot with only monitor 1 attached I can set it to 4K 60Hz. If I then add the other monitors, they all can run together at 60Hz. But if I then reboot. The monitor 1 at the thunderbolt output of the dock is back at only 30 Hz with no option to increase it.

I have all the latest firmware on dock and monitor installed now and I have also updated Thunderbolt and graphics drivers to latest version according to HP. The monitors are configured to prioritize resolution over USB hub speed. 

I also tried to swap the outputs or Monitor 1 and 2, because both are capable of USB-C video input. The effect is that Monitor 2, which is now the one connected to the thunderbolt output of the dock only gets 30Hz after reboot. I also verified that all monitors individually are able to run at 60Hz. So it is not the monitors. And I can get all together at 60Hz if I add them successively to the system. This setting does however not survive reboot. 

The laptop has two TB4 ports. I therefore tried to connect one of the 4K monitors to the laptop directly while the other two are attached to the dock. This does not lead to very strange behavior. The monitor directly attached top the laptop at the second USB-C port is discovered and all run at 60 Hz, but it is after a few seconds kicked out again and then rediscovered. This means it is not a stable setup. 

My point is that all three monitors were working well together in their highest resolution at 60Hz. This was working well for a long time and with successively adding the monitors to the booted system is still works. But in the past this did survive a reboot. Not it does not. What can I do?