Font Awesome - Not so much.

[Acorn]

Gary, the set of characters FA uses are not in the range of the usual ASCII set. The only one is SPACE.
All the FA symbols are instead input into a Text Area using the Insert > Symbol dialog.
You can input then as shapes if you have the Selector tool active.
If you use Character Map UWP then you can see and copy all 704 FA symbols and paste into a Text Area that is using FA.
Acorn

Nothing is broken.
If the supposed problem is the boxes then this indicates that the font doesn't have the particular characters. All of the characters in the FontAwesome font are in the "Private use" section of the unicode character range, they do not match with the "standard" characters...

I don't claim to follow that so could you explain why what is seen in xara is different to what is seen in affinity designer, are the actually different fonts ?
thanks
Edit - appears they both type letters, but xara displays symbols in it's font manager... hummm, well I'm no great user of fonts perhaps there is a reason... maybe acorn's post explains it

@Gary, it has always been like this, at least back to v12.
Xara Help:

So we have included the font, and an easy way to view all the symbols, as a way to incorporate the symbols either as a font in text or as raw vector graphic.

@Matt mentions FA's Private (Use) Area. For instance 'Glass' is U+F000, Private Use Area; 'Space' is U+0020, Basic Latin. The range difference is vast so what you keyboard will never be a FA symbol.
@handrawn, can you advise how to view FA in Affinity? FA has different flavours and it may depend on what has been licenced.
FA is a version 6. I believe Xara is still version 5. v6 has 2037 free icons. Not all are in the FA font I have on my system (c.f., figma).
The FA embed operation is web-based so it would fail for PDFs, hence the use of the FA Font approach. It would be nice to access the non-TT Font symbols to use then as vector.

Acorn