I'm still not sure what your setup actually is. But I am guessing you have the UIComponent set to Full Screen? If so, looking at the code the entities world transform is essentialy ignored for Full Screen UIComponents.
If you want to make sure this UIComponent is rendered above all other elements in the scene then you would have to change it's render group and modify your composition so that group is rendered last and that it ignores/clears current depth buffer.
I am currently not on a machine with Xenko on it so I can't give you step-by-step instructions at the moment. If you still are having trouble I might be back