I have added all the necessary sizes of my iOS App icon as PNG files so they have the correct masks. The Pictures show up fine in macOS and in the IDE:
But when the application runs in the Simulator, the App icon has a black border (see icon in Dock) ie treating it as if there was no mask. I did a forum search, but it seems no one has this issue.
Are your icons the exact sizes stated or did you leave space around the edges? Apple requires that they fill the space. In fact, you should make them square and let iOS do the rounded edges.