Hello,

To upload the custom italic font you need to name it Rawline-italic for example and set its font weight. Don’t need to set font-style for the custom font (there is no such field there).
Then, go to Typography settings and select your recently uploaded Rawline-italic for desired option and set its font-weight as well as italic font-style.
So you need to set font-style in the Typography settings only and not in custom fonts section. But you should have a separate font for this style called Rawline-italic.

Kind Regards