Comment 10 for bug 353090

It does appear to be a GTK+ quirk that is being triggered by translate_widget(). It occurs when the label is set for the GtkRadioButtons. This can be proven by skipping this step for widgets which start with 'login_', i.e.
            if not widget.get_name().startswith('login_'):
                widget.set_label(text)

I haven't yet been able to reproduce this in a simple test case yet. My guess is that setting the radio button labels while they are not visible (they are on a different tab) is not rendering the label when they become visible. As reported the labels get rendered when clicked on.

Note also the attached screenshots show one of the labels being truncated - not sure if this is related, I think it is a long-standing GTK+ bug but I haven't got the bug number handy.