Problem with defining corner BC/constraint

Hello all,

I'm trying to model the buckling of a shell structure under thermal load.

The structure is square-shaped.

The left edge must be kept straight. To achieve that, I applied MPC Tie Constraint for its nodes.

The bottom edge must be fixed so that the BC applied at its nodes was U1 = U2 = U3 = 0.

Temperature is applied as a predefined field.

However, when I run the simulation the corner that connects both edges deforms very strangely which makes me question how to model it.

I appreciate any input. Thanks.

PS. I've attached a picture of the simulation result.