Procedure to see this problem:

1. Start Scene Builder 2.0.
2. Drag 'GridPane' container to the center panel.
3. Drag 'HBox' container to each cells. 2x3=6 cells, order: (0,0),(1,0),(0,1),(1,1),(0,2),(1,2)
4. Now we can see 'HBox (0,0)', "HBox (1,0)", ..., "HBox (1.2)" in the Hierarchy Panel.
5. If I drag "HBox (1,2)" to between "HBox (1,0)" and "HBox (0,1)", "HBox (1,2)" turns into "HBox (0,0)".

This behavior doesn't be expected. We used to use this way in order to change the focus order of the cells in the GridPane on SB1.0.