It is normal behavior because it shows not only the current month but also the previous one and the next one.
You could try to use the ready callback to try to change the content of the header – the .jqx-scheduler-toolbar-details class could be useful.
But this is not a permanent solution because after the change of the date or the view you should change it again.
In addition, you could try to handle it in the rendering and in the rendered callback.
I would like to mention that this will be a temporary solution, not a built-in feature.