Solved by workarounds, post it here as a note for a chance to fix it by default.
Linux kernel 6.0.6-1 & 5.15.76-1
After today’s big upgrade, after boot it always shows only on built-in display.
If trying to switch to the external display, it fails, and it also fails to switch back to the laptop’s display (before the upgrade, it always succeeds to switch back to the built-in display, though operates blindly)
- When using only the built-in display, with to display mode “Unify outputs”, i.e. duplicate mode.
- Now both the internal and external monitor display normally.
- Switch to external monitor only mode, it works normally, even after reboot.
After waking up from long sleep (e.g. sleep for 1 hour), the same problem appears again, and it could be solved the same way.