Problem with display after standby [AMD Radeon]

I want to report here an issue with the standby mode under Kubuntu 18.04 LTS with an AMD Radeon HD 2400 XT GPU. The native “Apple” EFI based system, an iMac Aluminum, uses the very stable Radeon driver from Mesa.

Everything works almost perfectly, only the standby mode seems to have an issue after resuming from sleep. The display is corrupted. Well, in contrast to similar NVIDIA GPU related problems, the system can be still used. So it is possible to restart it, and after a reboot the display output looks again normal. Please check the attached pictures for more information.

For me this looks like a low level driver related issue. A refresh of the screen compositor doesn’t help anything. Only a reboot fixed it.

Update: I should clarify that also Kubuntu 18.04 LTS is running (like Mac OS X 10.11) in native EFI mode and not in legacy Bios emulation / CSM mode. This prevents the OS from certain limitations like non-AHCI, limited RAM, and other drawbacks.

Any help is welcome.