backlight not working prior to typing in password on startup.

Try a SMC reset and PRAM reset (hold the PRAM reset for a full minute so it activates several times).

Try creating another user account to see if the same thing happens when you reboot from the new user account. Try another round of SMC and PRAM resets after you create the new user. If it works, then you could try deleting the Display preferences from your home Library.

I do recall seeing this issue once before, but it was with an older model. I don't recall what the solution was for it. If a new user account doesn't change things, you may need to reinstall macOS. If you still have the problem after reinstalling macOS, then you will need to take the laptop to Apple or an Apple Authorized Service Provider to be examined.