Hello.
The field Display order is not easy to handle.
Better if you can do next thing:
After saving Entity field Display Order is reordering, so each other differs in 100, for example, there are 3 categories with display order: 100,200,300 . I insert new with display order 150, after saving all categories display order recalculates and become: 100, 200, 300, 400. new category has 200 display order.