When I built and ran my game on my iPad, the app icon is blank on the iPad.
As suggested by posters in StackOverflow, I have:
ticked the iPad option in Unity-iPhone > Images.xcassets > AppIcon > attributes inspector
added the icon into the slots for all the various devices
chosen 'Universal' for devices
selected No for 'Compress PNG Files' and 'Remove Text Metadata From PNG Files' in Build Settings > Packaging.
The app icon is a 512*512 PNG file.
What else should I try?
The problem has been solved.
It turned out that we must use icons of precisely the specified sizes. For iPad 83.5 pts 2x, we need to use an image of dimensions 167*167.