A new line appears on the edge of a surface when I use Shell. I can't figure out why. As I'm still officially a beginner, I have no fear in shamelessly exposing my shortcomings and begging for help online. In fact, I would be really happy if someone could educate me and point out what I'm doing wrong.

Picture 1: The shape as it is before I use Shell


Picture 2: After Shell (2mm / Flip).


Thanks!

Phil