I see you have selected Rigid as the connection between the four lines. You say RBE2 elements, but that is a NASTRAN term.  The ANSYS term is CERIG. Fixed Joints work by using Spiders from a node at the coordinate origin created for the joint. The joint is set to Rigid which is effectively making the entire rectangular pad rigid, adding stiffness to the model that you don't want. I see what you do want and in other software, that is easy to do. It's not so easy to do in Mechanical.

I assume you want to model these four edges being welded. I recommend you use Bonded Contact, Formulation = MPC to connect the four edges. This will connect the edges without adding stiffness.

If you want to add some stiffness to represent the weld bead, you can add a Bonded Contact, Formulation = Beam.
Note that you can define the Material, which could be a higher stiffness material, and the Radius of the circular beam section.

You will have also read my reply in the other thread.