FYI, A solution here half worked. Install the app. Restart the phone with the app still installed. After the phone has restarted the new icon is displayed. However, deploying from xcode seems to revert back to the previous icon until the phone is restarted again.

I have the same issue with an app currently in development. On new phones the new icon is displayed. But on my development phone (ios15) it does not update the icon. I've reproduced this issue by creating a brand new project. But keeping the bundle identifier the same as the old app. Running this new project on the phone the icon is the same as the old app. If I then delete that new project app off the phone, change the bundle identifier and display name it then shows the new icon. If I then change the bundle identifier back to the old app it then shows the old icon. This strongly suggests that it is nothing to do with the xcode project and must only be to do with the phone caching the icon. We haven't released this on the app store as of yet. Does anybody know if releasing to the AppStore with the new icon fixes this issue?