Hello, i have the exact same problem here with a similar environment (same Lenovo Legion laptop + Nvidia + XFCE), similar configuration files (main difference is that i’m using kernel 5.4) and same content in /sys/class/backlight when using acpi_backlight=vendor.
Did you manage to fix it ?
Is there a way to rename the /sys/class/backlight/ideapad to nvidia_0 ?
I could not use mv or ln -s on it.
I notice a version mismatch between mhwd -li (which gives video-nvidia-440xx) and the driver i am supposed to be using (linux54-nvidia-450xx). Could it be related to the issue ?