xticks should not be a double vector if you use datetime, it should be a datetime vector. I would expect anything else to return an error, but I could be wrong. Also, why are you changing the labels? Doing so will likely cause this behaviour. The date at the bottom right is a special datetime-type label. You can access it via
ax.XRuler.NodeChildren(1).String = 'test'
Messing with the labels might prompt the axis to remove that functionality.
Anyway, you need to show your script if you want a more precise answer.