I have put link rel=icon in the HTML with 48px icon size, tried with .ico and .png but the custom main site icon is not showing up in Google Search Result (Mobile). I have request reindex in Search Console, but no luck. I know I did request reindex so many times everytime I tried changing the link rel alternatives. Btw, I can see the icon on the browser tab and Google mobile shortcut thumbnail very well.
Meanwhile, I installed Wordpress in domain subfolder and set the icon, for just couple days the icons showed up. So I copied link rel=icon from WP to my custom main site, but no luck. It’s still showing default world icon.