Thanks for the reply 
I have been using the latest 4.5.1 build. So far this is the first asset that has done it, but it’s the first time I have tried it with a master material, even the tutors couldn’t work it out. I usually use OBJ’s to bake normals, but have recently started using SBM models baked in Xnormal, with all the smoothing groups split in the UVW space. I thought it might have been the green channel, but I flipped that and no difference.
It also seems to show up metal and roughness differently as well. As it worked for you it could be that there is an error in the project file or even the lighting as it shows up better if you shine a strong light at it, so I think I best test it in a new blank map to see if it still does it. I will
I have attached a containing the FBX file as well as the normal map. The model has a basic lightmap for now which will need sorting. Not the best unwrap in the world, but it’s split to smoothing groups, and will need making better for the final piece. Not my best model I have ever made.
I think it must be that I have done something wrong with the material setup though, as it looks spot on in all other programs. Could you also show me the setup that works for you? I can see where the error lies.
many thanks