While reviewing this I found an error in the style-linking.
The SemiBold Regular and the SemiBold Italic are in two different style groups.
They should be in the same style group. (nameID 1)
This means when SemiBold is being used, and the Italic button is pressed, it will not link to the actual SemiBold Italic font - it will be faked italic.
SemiBold Regular nameID 1 = Figtree SemiBold
SemiBold Italic nameID 1 = Figtree Semibold
So the Italic nameID 1 needs to match the Regular.
From @kenmcd in #41 :