Regression: Displayport over USB dock: Display can't be turned on since kernel 6.4

Hardware: Dell Precision 7740 notebook with AMD Radeon Pro WX 3200 GPU and Dell WD19DC dock, dell 30" 4k monitor connected by Displayport to the dock.

Software: Gentoo linux, kernel 6.4.1, xorg 21.1.8, amdgpu 23.0.0.

Problem: The monitor is recognized correctly when X starts, but is not enabled: xrandr says "connected" on DisplayPort-4 (as usual) and shows the correct modelines for the monitor, and the 3840x2160 line shows "60.00 +", but not "60.00*+". Enabling it with "xrandr ... -auto" fails in both 3840x2160 and smaller modes like 1920x1080. The monitor receives no signal.

xrandr error message: xrandr: Configure crtc 1 failed

X11 log error message: [ 94.572] (II) AMDGPU(0): Allocate new frame buffer 3840x2160 [ 94.572] (II) AMDGPU(0): => pitch 15360 bytes [ 94.587] (EE) AMDGPU(0): failed to set mode: Invalid argument [ 94.635] (EE) AMDGPU(0): drmmode_do_crtc_dpms cannot get last vblank counter

I found nothing in dmesg at the time of that error.

Exactly the same HW & SW configuration worked (and still works) in 4K@60 up to and including kernel 6.3.8 and fails since I switched to 6.4.1 (I tried no other kernel between 6.3.8 and 6.4.1).

The same monitor works in 4K mode with 6.4.1 when connected directly to the notebook with a HDMI cable (unfortunately, I have no DP cable to try a direct connection without the dock).