I'm running High Sierra 10.13.6 but my brightness still resets to full every boot.
I checked my Boot ROM and it's 16.16.295.5.12,0 which seems way higher than what was reported a page back.
What I think has happened is that I was briefly running my iMac Pro off an external drive running 10.14 Mojave beta.
It must have updated the Firmware and then the High Sierra 10.13.6 update didn't downgrade it again. So the brightness fix came in the High Sierra update but wasn't yet rolled into Mojave firmware.
I last ran Mojave beta 2 or so. Anyone know if the latest Mojave has this firmware fix?