I have a union with 2 sources and 4 fields.
I want a 5th field, and drag it over from one of the sources.
But I want this field to be on the top, so I open the union'er and move it upwards.
But afterwards, the order of the incoming arrows are unchanged.
The newly added arrow still points on the button field, which is now something completely different.
I suggest you modify the union'er so that moving fields also moves the arrows.