For an existing iOS app with a baseline of iOS 12, I already had an assets file the the full range of app icons. To include the 1024x1024 app store flavor, special 76pt (1x and 2x) and the 83.5pt (2x) images for iPad.
In the assets file, I clicked on the Mac checkbox and dragged in the custom artwork for all of those.
Now though, even after existing Xcode and doing clean builds, I always get three warnings:
But all those image are there and of the correct resolution. Individually selecting those images in the asset catalog shows the correct info in the properties panel. For example, for AppIcon_76pt_@2x.png, the idiom is iPad, Size is 76x76pt and Scale is 2x.
If I then uncheck the Mac checkbox in the assets catalog (and thus losing the Mac-specific artwork), the warnings go away.
I then turned on the Mac checkbox again and did a clean build. No warnings.
After then dragging in just the App Store icon (the 512pt @2x flavor) for Mac... hitting rebuild then produces the warnings. I will file a bug shortly.