What should I do if a 1pt black border appears around the iOS app icon?

Question

iOS

Friends, I ran into such a problem:
I draw an icon for an iOS application in Sketch, export it to PNG format in all the required sizes (@2x, @3x). I load into Xcode, run the demo on the iPhone screen - and it shows up with a thin 1pt black border around the icon, even though it doesn't exist in Sketch.

In Sketch, I do not use transparency and rounded corners for the icon, as it is written in the Apple guides. I can't figure out if the problem is on the Sketch side or already in Xcode? Please help!