Part of the issue, the largest part of the issue, is that Apple thinned down the thickness of the aluminum used in the frame of these iPads. That was my initial impression, without evidence, when I was thinking of returning mine. It was confirmed through closeup images taken on tear down sites, that delicately take new Apple products apart to show how they are assembled, and what they are made of. Comparing closeup images of the new iPad chassis with closeup images from the tear down of the 10.5”, and its chassis, it was pretty evident that the aluminum used was definitely thinner. Thinner aluminum bends easier. This is just my opinion, based on photo comparisons, and tactile feelings comparing the two models when I had them in hand.
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Hey OP I sent my iPad back for exactly the same reason, but the bend or whatever was only as small as in your second picture. It also made a slight clicking\ creeky(?) sound every time I picked it up in potrait mode (left hand corner) and tapped against the backside with my fingers (right hand less firm and tapping), where the spot is (see other threads with creeky sound or weird noise issues).
I sent it back as well (Germany -> Widerruf) which was supposedly successful (though I don't have my money back yet).
I swear I didn't sit on it, I used it very normally in the three days I had it. No situation that I remember where I put extensive pressure on it. I've possesed 4 iPads in my life (iPad 4, Mini Retina 2, Air 2 and iPad Pro 10.5) and I've never had this issue. In fact I've treated them much worse over their (later) lifetime while I treated this one like a baby!
Edit: Also I wrote "bend" instead of bend because I think it was not exactly bend. Like in your pictures, it didn't exactly go to the edges, so I'm not sure what it is. Maybe it's coming from the empty space below, which can be seen in one of the pictures that someone posted here.
You sound like you experienced exactly what I did, and what was one of two deciding factors in returning mine. When held in two hands, in portrait mode, I could delicately flex it, and it would click/pop. The best I could compare it to was the noise the little button makes on metal lids to glass jars that allow one to test if the jar has been opened. They sort of click/pop. That is exactly what the iPad and its large flat aluminum back did, click/pop when flexed lightly in two hands, in portrait mode. Out of dozens of iPads I’ve owned over the years, this was the only one I have ever had do that. Every other iPad doesn’t flex, and feels like a solid, well built piece of metal and glass.
