I think it's a missing feature involving being able to move a stacked object up or down the stack.
I have seen a similar thing happening when overlaying a Label over a button on a panel (to create buttons with colored text). It's a layering effect when stacked controls are placed over each other. In my case, the Labels need to be at the top and the Panel needs to be at the bottom with the buttons in between. When you select a button, it moves to the top. When you get them in the wrong order, you must select the one you want on top and double click or move it slightly to bring it to the top. You likely will have to set the Design Lock to prevent selecting the wrong item.
In my example, the Panel is Always in Back. If I need to change buttons, I lock Panels, Labels, and sometimes Shapes. Then select and modify the buttons. When finished I lock all but Labels, select the Labels then double click to bring them to the top.
When a button is not on a panel, Send to Back works, but not when the button is on a panel, which is always in back.
Some Graphic type programs with Layers have controls to move a layered object up or down in the stacking order. This would be a nice new feature for MDP.