There is a bug with the extend map option during the runoff votes. When the runoff vote is shown for the first time, it shows the 2 maps that are supposed to be voted, and under them it shows the option 6 for extending the current map. This option is invalid, you cant select it and it disappears after you select one of the 2 maps.
So i think the bug is with the menu display for the runoff vote, the first time. After you select one of the runoff maps, it continues working and displays the menu properly.