Two good suggestions and a method towards diagnosis. Sorry for the delay; I'm not always here.

  It is a set that I bought. The mirror looks flat, but may not be. Not sure what is meant by ' a carefully designed normal map applied.'

 I did 'borrow' the reflections material from a mirror that was working on another set so I tend to suspect the mesh here.

I'll see if I can get a screen capt of the mat room settings. I'm wondering now if I have a spherical reflection map applied someplace and it is inverting?

Also I will see if I can separate out the mirror mesh - or turn it off independently - and replace it with a poser plane prop.