This issue appears to be fixed with XCode 13. Metal libraries compiled in XCode are now working on iOS 10 for us, so you may want to remove your workaround.
Columbo
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I think the problem probably lies with the XCode packaging options rather than the Metal API.There's an option in `Build Settings`->`Compress PNG Files - Packaging` called `Compress PNG Files` which recompresses your PNG files with a custom version of PNGCrush, amongst other things (switching to BGRA IIRC), it premultiplies the alpha.