When using a scale bar with the unit of measurement below the bar, the graphic element box should reduce in size so that centring the element provides the expected result. Currently the element is much wider than it needs to be and results in the scale bar not being able to be centred properly when using the align option on the contect menu.
The image below shows the horizontal centre of the element, which is almost 25% of the length of the scale bar away from the actual centre of the scale bar (the centre point shoulf be at the '4' division).