Dashicons and Font Awsome

You probably have some other plugin that is loading it’s CSS and/or fonts on all admin pages instead of only their own Settings page and that is probably messing with WP External Links icons.

I found only one Plugin which uses Font Awsome and Dashicon. But disabling it makes no difference.

Since the icons are showing when selected but not in the dropdown, it can be CSS unrelated to the icons but to the dropdown ??

This is how it looks.