Not enough space allocated for widgets that was invisible
In the network panel of GNOME Control Center, when you open the details dialog of a ethernet connection, by default "bytes" is not visible. And it became visible when you change the MTU value to non-automatic. But then the "bytes" label overlaps with the spin button. And the allocated width "bytes" label is 1(screenshot 1).
I tried to debug this using gobject-introspection tool. As soon as the introspection tool is shown, the "bytes" label gets enough allocated space(see the terminal output in screenshot 2).