If anything, Firefox on Linux is more likely to get it right, as it will typically be shipped with the font Twemoji Mozilla which includes this flag. I think they don’t ship that font on macOS or Windows, and it sounds like at least the default emoji font stack for Windows doesn’t support this flag.
Most flags are made of two regional indicator symbols, like this for the United Kingdom which has ISO 3166 code GB:
U+1F1EC REGIONAL INDICATOR SYMBOL LETTER G
U+1F1E7 REGIONAL INDICATOR SYMBOL LETTER B
If the font has flags, it should render this as a Union Jack, but if it doesn’t, it should render something along the lines of “GB” (which, frankly, is generally more useful if you try using flags without labels).
But Scotland has the ISO 3166 code GB-SCT, a format the regional indicators weren’t designed to cope with (for better or for worse), so it uses a different encoding technique and is comprised of the following sequence of seven scalar values:
U+1F3F4 WAVING BLACK FLAG
U+E0067 TAG LATIN SMALL LETTER G
U+E0062 TAG LATIN SMALL LETTER B
U+E0073 TAG LATIN SMALL LETTER S
U+E0063 TAG LATIN SMALL LETTER C
U+E0074 TAG LATIN SMALL LETTER T
U+E007F CANCEL TAG
As you observe, this has a significantly worse fallback behaviour: the tag characters are non-printing, so you get just a black flag.
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.
Both font awesome fonts