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A1990. External display works normally when eDP is unplugged. With eDP connected looks like this. Potential gmux issue? No signs of screen damage. Have yet to work on this model yet.
Looks like this one still has Apple Care on it. Going to have customer contact Apple. But still curious what the issue is. I believe I saw Rossmann do a video where the eDP connector had debris on the backlight and data line that killed the GMUX.
Thanks Duke. The only reason I didn't suspect GPU is because with the eDP cable unplugged, using an external monitor, there were no graphical anomalies. Thankfully, this one still has a warranty until 2022.