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I tested your case with using a custom font in the project while using the latest NativeScript 3.3, however, was unable to recreate an issue while building it for iOS.
For your convenience, I am attaching sample project and Gif file, which demonstrates that the font is applied properly. In the project, I set up the font family, while using the font name.
Regarding the attached project in your comment, I notice that the application refers NativeScript 2.5. I would recommend testing the same case with the latest version of NativeScript