I'm still trying to pinpoint exact driver <-> Blender version combination that was working better than current release and AMD drivers. It will take me couple more days I think.

Whoops. I forgot to include working version.
Now that I'm trying to test, this issue is present even in 2.83.10 LTS! And I'm unable to find Blender version in which hybrid OpenCL rendering don't have CPU underutilisation problem.
I remember it working without a problem in 2.90.1, branch: master, commit date: 2020-09-23 06:43, hash: 3e85bb34d0d7. In the meantime I upgraded GPU drivers and Linux kernel and after that this release also have this problem.
Pasting old system-info.txt:

Can confirm, it's working. Thank you!

It's hard to tell from the image you attached, becase of the feature you chosed. In my 4th picture features on both images have the same size but different resolution.

Thank you for checking. Please don't close it yet.
I assume you didn't change image output size from 19200x10800px?
If you can please save the rendered image and compare resolution of the image inside image editor in Blender with the resolution of the saved image. More feature-rich scene might be better example.

Want to point that alt-click-drag is commonly used in many Linux distros for moving windows around.
It might be good idea to take this into account when assigning shortcut for this functionality.

I've run some tests and it seems that regression is happening in hybrid CPU+GPU rendering also.
Unfortunately I cannot pinpoint exact scene config for solid example.
Default cube renders without regression, but on production scenes with heavy geometry with displacement, volumes and lots of lights (both emissive mesh and standard) I have dips to 0% CPU utilisation.

After uninstalling AMDGPU-PRO drivers the hangups are gone. I've succesfully run animation render overnight and baked smoke simulation.

Most of the hangups are happening in 2.90.1, as this is the version I'm running for production right now.
But it was released 23. Sep - way before multithreaded geometry export patch (21 Oct).

@Ankit Meel (ankitm) suggested in DevTalk thread https://devtalk.blender.org/t/blender-randomly-hangs-what-cli-debug-options-should-be-enabled-for-gpu-issues/16660/3 running Blender with only one thread (-t 1 argument) to further troubleshoot.
I run animation render overnight that way and there's no sign of hangup so far, and Blender is still responsive.

Thank you!
After quick test the CPU looks fully utilised in all above scenarios.
As for exact performance numbers I'll need to check out. Nevertheless most regression seems to be gone.

Thank you. I'll try to fiddle with drivers and find some solid information.

After https://developer.blender.org/rB2e1498ff16198742ba543004fd9c2c49083a6095 patch I'm no longer experiencing crashes with rendering which is nice.
I've done some tests on current build hash 010f44b855ca branch master.
The bigger the render image size the bigger the performance hit.

Thank you. I want co confirm, that on recent builds the problem persists - after launching Blender from command line I got segfault as before.
The crash occurs straight after launching render. There is no error message either.

Thanks for checking this.
I should have mentioned that I dont use GPU for rendering - Cycles on CPU only.