Initially, there are errors about backlight
# systemctl --failed
UNIT LOAD ACTIVE SUB DESCRIPTION
● systemd-backlight@backlight:acpi_video1.service loaded failed failed Load/Save Screen Backlight Brightness of backlight:acpi_video1
● systemd-backlight@backlight:amdgpu_bl1.service loaded failed failed Load/Save Screen Backlight Brightness of backlight:amdgpu_bl1
The error has gone by adding acpi_backlight=video to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub & update-grub
The problem left: backlight of laptop screen always on when display on external monitor only
(of course the backlight will be off if the lid is closed)