In the kernel logs, an error is shown :
[ 2.133074] [drm] Display Core initialized with v3.1.59!
[ 2.148452] [drm:dc_link_detect [amdgpu]] *ERROR* No EDID read.
Reverting back to 4.18.16.arch1-1 fixes the problem.
It seems something is subtly wrong with monitor communication.
This problem has already happened with a previous kernel.
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