Well it would appear that High Sierra has a different version of com.apple.driver.AppleBacklight. So that is a clue.
com.apple.driver.AppleBacklight (170.10.1)
com.apple.driver.AppleBacklight (170.9.20)
Maybe upload a copy of both of those Kexts and we can dig into what changes have been made.
I am on Sierra currently so i can grab the 9.20 actually. So if you want to just grab HS that should help.