UV distortion, apparently some scaling/dimension issue because unwrapping after making the mesh 500 times bigger fixes the problem.
That's a simple one-sided curved object (7 faces) located perfectly straight (pic. 1). So I expect UVs to be perfectly straight as well. But it gets slightly angled (pic. 2) which is critical for my tiling texture. Making the mesh much bigger and applying the scale makes the unwrapping process behave right and expected (pic. 3)