A shell model is an idealization of the true structure and part of the idealization is choosing what features to be approximated more than others. There are many ways to build a shell model of this structure. They will all have similar global stiffness behavior, but details of peak stress will differ depending on how the idealization was done. 

There is a true stress concentration due to the sudden doubling of material thickness at that edge.

There may also be a stress singularity so that the stress will increase without limit as the elements get smaller.  Use a Mesh Sizing control on that edge to create smaller elements and build a series of results to plot peak stress vs element edge length. Please reply with that data plotted out.