This is a known issue. Acetate objects of an ArcMap Service, such as north arrows, scalebars and text, may display differently than an Image or Feature Service. This also occurs with acetate lines and polygons when changing the coordinate system of the ArcMap Service.
Another example occurs when adding an acetate layer object and specifying the coordinate units as 'database'. Selected features, highlighted in cyan, appear on top of the acetate object when using ArcMap Server. When the acetate layer object is added with coordinate units specified as 'pixel', the selected features appear below the acetate layer object.
There is no workaround.