Thank you for doing the test! I described what I expected to happen, based on descriptions in the ANSYS Help system like this...
so it is interesting that it did not work the way I expected.
From your test, my new understanding is that there must be an algorithm that decides when a node has "enough" connections and stops. I don't know for sure.
One part of the test, which is not important to do as it only can cause a problem, was to set the Pinball radius small, which you did, but that only works if there are an odd number of elements (like 11) across the width, which leaves no nodes at the centerline and makes a gap between the nodes on the vertical and nodes on the horizontal. You have 10 elements across the width, which create nodes along the centerline and no gap.
These MPC Constraint Equations are described in Chapter 15.14 of the Theory Manual of the Help system, which I don't find that helpful. MPC stands for Multi Point Constraint. This creates a set of equations, using Lagrange Multipliers, to enforce a set of requirements on the kinematics of the set of nodes on either side of a contact. MPC is similar to RBE3 elements that distribute a force to a set of nodes without adding any stiffness. This is in stark contrast to the CERIG elements that define a rigid region which is used in a Remote Displacement or a Joint when the Behavior is set to Rigid. This does add stiffness to the model (NASTRAN calls it an RBE2 element). Remote Displacements and Joints can also have the Behavior set to Deformable, then it uses RBE3 elements that don't add stiffness.
The default Formulation for Bonded Contact is Augmented Lagrange according to the Help system. The reason I like MPC is because I can do a visual check of where the contact may be missing in some areas and I can increase my pinball region to cause it to connect.
Here is another test you can try if you want. Make a cylindrical sheet body at the center of a flat square plate. This will create conditions where there is a large variation on the distance between nodes on the contact side and nodes on the target side of the contact. Play around with element size on each body. Flip the Target-Contact side of the contact by a right click on the contact. Try a number of different Pinball Radius values to see if you can get an intermittent bond around the circle.