is it original game model? there was wrong bump map appearance on original vehicles after being imported into zmodeler; shaders were updated to render bump properly. So, if this is not a stock model and you made bump map to look properly in older version of ZM, you should rework bump bu either flipping or inverting red and blue channels.
I've tested bump on "doorshuts" material of buffalo model. it's absolutely ok being loaded original vehicle, after using normals calculate in ZM (no visual changes, so normals are loaded ok) and after export and re-import.