This is not coming from Font Awesome; you’re using regular emoji, and depending on the host being able to render it. U+1F355 SLICE OF PIZZA has been around since 2010, but the Scotland flag sequence is from 2017 and some platforms have been deliberately not including country flags because they are generally not necessary (though the Scottish flag suffers more than most because of its composition, see my other nearby comment) and they are the subject of a few international disputes (e.g. PRC objects to Taiwan’s flag being included) and so it’s easier to just keep out of it all.