editing category in container changes menu display

I have a Container with 3 child categories. I want the Container to be a child of the Primary Links Menu, and each category to appear under the Container parent in the menu. When I create them, all is well. But, if I edit a child category's body, and change nothing else, suddenly the child category appears as a menu item at the same level as the Container, its parent. None of the settings has changed, just the menu behavior. The child category appears as a text link in the container, but not on the Container's menu of children.

SETUP:
CONTAINER A
CHILD CATEGORY 1
CHILD CATEGORY 2
CHILD CATEGORY 3

Now, edit Child Category 2's body only, to add some text.
NOW:
CONTAINER A CHILD CATEGORY 2
CHILD CATEGORY 1
CHILD CATEGORY 3