Simple sweep. Why this when I cap?

Usually when I see odd results, I can trace it back to something funky in the geometry. I can’t see why this happens?
A simple Sweep 1. Looks fine. Then I cap or join an appropriate face at the bottom and this happens?

Why.3dm (201.0 KB)

Hello - use DivideAlongCreases > SplitAtTangents=Yes to clean up the display in shaded modes.
This happens because the mesher does not like the tangent condition in a single surface there where the arc is - if you split it at the tangerncy, with that command, the mesh knows what to do.