Problem with world normal

This is driving me nuts. I have two panels which are part of a modular wall. The panels are identical except that one has a doorway cut out of it. They both use the same texture and have identical UV maps. But the world normals are coming out different which is messing up the normal mapping of the wall. If you look at the picture, you can see the world normals are different on either side of the black line which is where the two meshes butt up against each other. There is no scaling or rotation on either panel. What is going on here?

Having a really hard time discerning what’s in the image so here’s a best guess.
The walls are on a different plane. Either skewed in the modeling program, or a float point on the world rotation got rolled over one way. Or one of the walls faces is inverted; test by turning on back-face culling in the editing software.

Those aren’t the issue. The modeling program (Blender) displays a rude color on faces which are facing the wrong way. I can apply the colored/numbered grid pattern on the two meshes and they match exactly. The two meshes where made together. The only difference is that one has a door cut out of it. Otherwise, they are the same.

It does seem that the face normal is pointing the wrong way. However, if I set the material color to display the PixelNormalWS, everything looks ok.

In blender, make sure the scale is 1,1,1 and not inverted on any axis like 1,-1,1. You can use “apply scale” function to fix, or manually input 1,1,1 to scale.

Be sure to check back-faces anyways. In the current version of blender (2.91.0 at time of writing), back-faces are undiscernible in all shading previews when using flat shading. When using smooth shading, it’s only discernable when some (not all) the faces are inverted as the shading will appear warped.

Scale is all 1,1,1. You can discern back faces by checking the Face Orientation box under the overlay menu.