I have the same issue but I have 2 mid-2009 17" MBPs and one of them has the backlight issue and the other one doesn't although I did install the OS a bit differently on them. See my posts above on this page.
Onyx Maintenance script didn't do anything for the backlight although I'm not sure if its supposed to.
Hm, I see. Well that's weird. I noticed you also formatted to APFS. I was considering that but decided to stick with HFS+ just in case. Looks like it was a pain in the ass. Did you format both your mid-2009's to APFS?