It looks like the issue lies somewhere at the intersection of the amdgpu driver and Mesa: the problem doesn’t appear in the kernel console, only when a desktop environment is running - and all of them rely on rendering acceleration via graphics APIs.
A working fix that doesn’t require a kernel rollback was suggested in a related thread: change the “Color accuracy” setting from efficiency to color accuracy. In KDE, this option can be configured under “System Settings → Display & Monitor”.
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