Custom fonts not working in TestFlight app

I'm stumped. I have been trying to include a custom font in my app, but on the version released to TestFlight, it is replaced with Helvetica.

I've followed all the steps on https://developer.apple.com/documentation/uikit/text_display_and_fonts/adding_a_custom_font_to_your_app .

The font shows fine in both testing and release builds on Xcode, on both my device and simulators, its just the TestFlight build which doesn't bundle it.

Any help would be appreciated :)