Aniso shading error.

Observe what happens in the following snapshots.
Normal Z Axis Radial tangent works fine and what I’d expect:

But when I use UVs to map tangent, I get this horrible sharp shading artifact:

For this specific object on its own I could use Z Axis Radial, but I have other meshes which needs to be UV controlled. I just used this as an example.

Note how the shading “light tail” doesn’t stretch over the seam. Is this a known bug? If not, can someone report it? I don’t know where. Or am I doing something horribly wrong?

This happens both for GGX at “normal ranges”, as shown in screenshot, and for Beckman at full iso and roughness way above 1. The anisotropic Beckman seems quite a bit odd in how it works by itself (like, roughness > 1 required for full aniso effect, and negative aniso just acting crazy).

I cannot find any problems with normals, it unfolds fine, and no merged pole points, and rotation & scale have been applied. Tried everything I can think of.

Have you tried to scale your UV map in the x axis, so that both edges, right and left, stay inside the UV layout?

I’ve tried everything within, with boundary and pixel snap. Even slightly smaller and also much smaller. X and Y and both.
None of that had any effect. However:
Scaling to a size way smaller than a pixel but still greater than zero, appeared to have fixed it.
But turned out it just relocated. Going even way smaller to this introduced really weird stretching artifacts, but I guess the values just become too small to calculate an angle.

Does other software have the same issue?

You might want to report this to the bug tracker, as it looks clear to me that it might be an error in how Cycles calculates the tangent.

Otherwise it may not get fixed.