Custom fonts stop showing sometimes when you switch applications in SwiftUI

I have loaded custom fonts into my SwiftUI project in Xcode to a folder, with the target membership set to the correct build, I have included the correct names inside the info.plist in the fonts provided by application section, and included them in the code using the correct names as such:

extension Font {
    static let mainFont: (FontWeights, CGFloat) -> Font = { fontType, size in
        switch fontType {
        case .hairline:
            Font.custom("TT Norms Pro Thin", size: size)
        case .thin:
            Font.custom("TT Norms Pro ExtraLight", size: size)
        case .light:
            Font.custom("TT Norms Pro Light", size: size)
        case .regular:
            Font.custom("TT Norms Pro Regular", size: size)
        case .medium:
            Font.custom("TT Norms Pro Medium", size: size)
        case .semiBold:
            Font.custom("TT Norms Pro DemiBold", size: size)
        case .bold:
            Font.custom("TT Norms Pro Bold", size: size)
        case .heavy:
            Font.custom("TT Norms Pro ExtraBold", size: size)
        case .black:
            Font.custom("TT Norms Pro Black", size: size)
        }
    }
    static let serifFont: (SerifFontWeights, CGFloat) -> Font = { fontType, size in
        switch fontType {
        case .semiBold:
            Font.custom("Butler Free Bd", size: size)
        }
    }
}

extension View {
    func mainFont(_ fontWeight: FontWeights? = .regular, _ size: CGFloat? = nil) -> some View {
        return font(.mainFont(fontWeight ?? .regular, size ?? 16))
    }

    func serifFont(_ fontWeight: SerifFontWeights? = .semiBold, _ size: CGFloat? = nil) -> some View {
        return font(.serifFont(fontWeight ?? .semiBold, size ?? 16))
    }
}

Most of the time this works and the font shows, but if I switch applications, often some of the fonts disappear and the standard iOS font shows. Sometimes, but rarely, it also already doesn't show correctly on launch. I also tried simpler ways to include the font and different font files, but it causes the same behavior. What could be the reason for this erratic behavior?