Thanks. You have a filter on in the tree which is hiding meshes, so once you make the region attachment into a mesh attachment, it disappears from the tree. When the mesh is not in the tree, the tree won't display its properties, even though the mesh is selected in the viewport. The tree has the slot selected, so it shows the slot's properties.
You can right click the red filter icon in the tree to toggle it, or left click to see and change the filter settings.
