I have the same problem, even if the retainer material has nothing in it, but the source texture. Hell, even if I don’t assign a material at all! Just the plain old retainer box wrapping an image: the image gets bright and washed out. I’m using it in a 3D widget in the VR resource project, MotionControllerMap. I’ll test it in a completely empty map …
Edit: It is a known bug in the Engine as of 4.18p2: https://answers.unrealengine.com/questions/712257/418p2-retainer-panel-causes-textures-in-3d-widgets.html