Question What causes my normal map to be inverted on certain faces Unity URP + Blender

I have been struggling with this for a while and can't seem to find out how to fix it.
I have modelled this boat in blender and made a normal map in substance painter. I exported the normal map and imported straight into Unity using the urp metallic standard output template. To my surpise certain faces are inverted when shading it in Unity.

I have no overlapping UV's in the problem area

no inverted faces (the blue image, should show red if inverted faces).

There are no broken vertex normals in blender though in this image you can see shading artefacts already.

In substance painter there are no shading issues.
I have set the imported texture is checked as a normal map. From what I can see it should be something mesh related but what it is. No clue.

I am using the urp and this is the normal map I have used.

 

That does look like a broken vertex normal for the affected polygons (where you say you see shading artifacts). You ruled out inverted normals but not continuous normals. It might be the difference between "shade smooth" and "shade flat" which end up giving different smoothing groups / split vertices in the import. Or it might be a ripped edge, which can cause the UV interpolation to be different. I would try this in Blender: go to object mode, apply scale, go to edit-mode, vertex-mode, merge-by-distance, face-mode, select-all, recalculate-normals-outside, shade-smooth.

 

That did not seem to be the solution. It was still broken. I now deleted all the problem areas and remodelled them, it seems to work now. Though the problem is still a mistery