How do I add an icon for an installed application to one of the MIUI app icon screens?

My friend is using a device running MIUI 12.5.1. They have installed some app, and then some kind of magic happened, in which they were told by the device that it would be cleaning up / removing icons of unused applications - even though the app had been used; and the icon went away. Now, they want to restore the icon. The app appears to be installed when examining Settings | Apps | Manage Apps. My friend reports they can't find the icon anywhere, i.e. hasn't just been moved (although TBH when they told me this I got suspicious it's in some folder they're missing). How can I restore the app icon? I've searched the Settings, and have not found anywhere this might be possible; app screen customization also only offers widgets, not app icons.