When trying to add to the site icon to the home screen of a mobile phone with Google Chrome we get a grey square. All sites where we we have not upgraded to the latest version of WordPress the icon appears correctly.
I have looked at this further and found the cause of the problem. It seems to have been caused by the plugin “WP Content Copy Protection Premium (YOOPlugins) Version 1.8.32” This seems to have a conflict with WP 5.0 specifically – and prevents the Site Icon from being added to the mobile phone home screen. When we turn off the plugin all is back to normal.