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Overheating sounds like a possible cause for this type of corruption. But in my case it would have to have been the heat that made a permanent fault. It happens even if the unit has been unplugged for some time.

I finished Zelda with 96 hours of play in 2 weeks, so it's strange it didn't overheat then. But I suppose at that time it's also protected by a fan...

Yeah. Apparently it is permanent damage to the GPU. Article here.