This is the expected default behavior; by default text objects have their Clipping property set to 'off'. The Clipping property controls whether the text string is clipped when the text is outside the axes limits. (The axes properties Clipping and ClippingStyle control whether and how clipping is done, respectively.)
To avoid this, set the texts' Clipping to 'on', e.g., when you create them. (And you may want to set the axes ClippingStyle to 'rectangle', which looks better than the default '3dbox' on 2D axes, in my opinion, because it allows for partial clipping of the text, rather than "all-or-nothing".)
