This is not a bug, it's by design. See the manual entry for Text:
ote if WebGL is enabled in project properties, due to technical limitations all Text objects appear on top of everything else in the game. For example, Sprites cannot be displayed on top of Text objects when WebGL is on, because the text is rendered to an overlay above the game. It is recommended to design your game so it is not necessary for other objects to appear on top of text objects, since turning off WebGL can reduce performance.
IE9 does not support WebGL, which is why it Z orders as you expect.