If you exclude the elements along the fixed support edge, the stress is very close to 12 MPa, which a hand calculation of Mc/I shows is the correct answer. The simple Mc/I equation applies to beam elements that have only a single node that is fixed.

Shell elements have a row of nodes along the fixed edge. The material has a Poisson’s Ratio. The value of Poisson’s ratio is the negative of the ratio of transverse strain to axial strain. Because the nodes are fixed in the transverse direction, the transverse strain causes an additional stress to the bending stress you calculated. Try changing Poisson’s Ratio to 0 and you should get a more uniform stress result. Alternatively, you could modify the support so that the nodes along that edge are free to move along that line, except for a single node that is fixed.