I haven't found this covered anywhere, it it was, I apologize.
ExtJS 7 introduced new FA flavors, namely version 5 of solid, regular and brands.
That works well anywhere iconCls is allowed but doesn't seem to work as a glyph reference.
And a "fix" is to add single quotes around the name of font family after @ sign. It has to be single quotes because of how font family is passed through to a style attribute.
Here's a fiddle demoing this issue: https://fiddle.sencha.com/#view/editor&fiddle/30i4