GBowler 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.
I wonder if you can compare your non-symptomatic MacBook to the symptomatic one and report back further differences? The devices have identical configurations and exact OS versions? You reported already the different firmware versions; I am also curious about any GPU differences in the System Information app (especially ROM revision, gmux version, EFI driver version, etc.), SMC/BridgeOS version, the specific EFI firmware versions, etc. Anything you can observe/report may be insightful. I would love to pin down the specific update causing the issue, and whether it may be reversible.
Also see this MacRumors post about one user's eye strain which began with a specific subversion of macOS Catalina.