First time I’ve seen this issue mentioned in any review about the Air.
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All quotes from this review.
m.gsmarena.com
More worrying is that we found the build quality to be less than perfect and the overall feel somewhat cheap, despite the premium materials. There's a bit of subjectivity to that so we would suggest you try it yourselves instead of blindly trusting us on this one. But if you are on the same page that's certainly going to be a deal-breaker for a design-first premium smartphone.
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But while the engineering side of it is downright impressive, the actual in-hand feeling of the Air left us puzzled. We can't quite put our finger on it, but the side frame's finish feels cheap, almost plastic-like. The gaps where the side frame meets the front and back glass really don't befit a design-first product. And that glossy element on the camera bump just looks and feels off.
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All in all, the body is impressive, but the design decisions are a bit polarizing. The chosen materials are excellent, but they are not put together as well as they could and overall the Air just doesn't feel as premium as one would expect. On the other hand, there aren't that many premium, lightweight options available right now, and it's the thinnest iPhone ever made (excluding the camera bump), so that will surely count for something.
All quotes from this review.

Apple iPhone Air review
The popular Air series finally reached iPhoneland. A momentous occasion that Apple deemed worthy of a one-off naming and omitted the "17." After two less...