QTableWidget header item alignment defaults to Qt::AlignHCenter|Qt::AlignVCenter visually but Designer shows AlignLeft. This is because it is overriding this somewhere if it is not explicitly set so that it is aligned in the center for a header item.

Either the default (where it is set) needs to be changed to reflect that it is AlignCenter or Qt Designer should see it as being AlignCenter by default and not AlignLeft.