macsforme I have been investigating reports of eye strain on MacBooks being triggered by a firmware/OS update. I also bought a few tools to dump and flash different firmware versions on MacBooks (specifically testing the MacBooks I have, which are mid 2015 15-inch Pro models, both with and without the discrete AMD GPU). So far I cannot pin down the issue, but there have been several reports like yours and I plan to keep investigating. A GPU VBIOS update seems the most plausible.
This might not be possible, because it requires a "known good" system of the same model but in that case it might be possible to compare firmware dumps between a "bad" versus "good" system and potentially identify what's different.
From what I remember (almost) all laptops integrate the GPU VBIOS into the UEFI ROM image, as opposed to desktop GPUs where the GPU VBIOS is flashed onto a separate chip, so identifying or changing the VBIOS in use can be done with external tools.