Found the issue.
This isn't a problem in atan2, this is a problem in how the GPU finds which MIP level to use in the texture. If you connect the MIP input of the Texture 2D node, the artifact goes away :)

This has now been added in 1.18

Glad you like it :)

I've wondered this as well recently, I'll look into it

I'll look into this tonight :)

I can't reproduce this, the bug doesn't trigger for me :(

This has now been implemented in 1.18

Oh, that's super useful, thanks! I'll implement this in the next update of SF :)

The code node doesn't allow code outside the local scope of the fragment or vertex shader, so I'm afraid that's not an option either :(