Does the backlight control itself work like before? The backlight going darker at the boot time might be the systemd trying to restore the saved value. See systemd-backlight man page. It's saved in /var/lib/systemd/backlight/*. And the backlight going darker during X session is done by the desktop itself. I haven't noticed a problem on a similar machine (Latitude 7270) but with XFCE, so this might be depending on the desktop.