Did you have your iPad outside at full brightness? That's the only reason, IMO, for needing to use full brightness. Combining that with a thick rear case would mitigate the cooling properties of aluminum. The sun beating down on the front combined with the ambient temperature might be too much for such a slim thermal profile to handle, especially when you consider that full brightness on the newest iPad Pro is now double that of its predecessor. That would be unfortunate as, like I said, the main use case for full brightness is outdoors use. I've been meaning to try mine outside, but it has been so hot lately. I also don't have a rear case on it yet, so I'm worried about dropping it.
