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I know you don't want to hear this but it's odd that it is always the same area, leading one to believe that it is environmental. The way it is being placed in a pocket, the way it sits in that pocket, tight jeans, loose jeans, how it is being carried, where it sits during charging, something environmental seems to be affecting the device in a way that causes that corner to lift. Having used the 6 - 8 all with the same issue the odds of getting one device from each run that has a similar defect, out of millions of devices, very low. I would also find it difficult to think you weren't cleaning that same corner when applying all of those screen protectors, or that you've had a rash of bad protectors all with the same defect. Were they all from the same MFGR? What about cases, as Bkodger1 mentioned? Are they all the same type or from the same MFGR? As someone who's profession is as a troubleshooter I can tell you there is no way to produce that level of consistency randomly. If the same issue presents itself again and again there is a definite specific cause. If you said it popped up in the middle sometimes or in other corners, I would still believe it was environmental but identifying the cause might be impossible but this, the same corner, presents in the same way... One should be able to point that out after a few typical days of keeping track of the device.
Let us know what you come up with.