@joshfriesen going by everything you have checked and tried, this does sound at minimum like a failed LCD or at worst a failed component on the motherboard for the video. At this point, you have to make a decision to either replace the screen, or to return it to your customer as "not repairable"
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Of course, you can always check the components and see if that reveals if it is the circuitry or the display. Since you did not tell us the exact model for this tablet, here is the troubleshooting for the SM-P615/619
@joshfriesen going by everything you have checked and tried, this does sound at minimum like a failed LCD or at worst a failed component on the motherboard for the video. At this point, you have to make a decision to either replace the screen, or to return it to your customer as "not repairable"