Editor cannot render some unicode symbols

Qt Creator seems to have lost the ability to render some unicode symbols in the code editor.  Using 'Ubuntu Mono' in Qt Creator displays:

Whilst using the same font, on the same machine in VS Code:

What may be the cause is that on another machine using the same OS, same Qt Creator version, but running Wayland - the symbols render correctly in Qt Creator.

I've no idea how to debug any further as a user!