SwiftUI buttons change text size at appear

As can be seen in the GIF, the "Done" button text becomes larger as soon as the sheet is completely open. This not only happens in this view, but also in others which use systemimages instead of text. Does anyone know a solution to the problem or do I do something wrong? I'm still relive new with Swift.

    NavigationView {
        Form {
            ...
        }
        .toolbar {
            ToolbarItem(placement: ToolbarItemPlacement.navigationBarLeading) {
                Button(action: {
                    self.showSheet = false
                }) {
                    Text("Done")
                        .bold()
                }
            }
        }
    }