Overridden items from parent pages are not really overridden, and may be silently deleted. Here is how:

1. Make new spread document
2. Go to Parent Page "A-Parent"
3. Make a red box on left master page. Make a green box on right master page
4. Add two pages to the document, page 2 and 3 is now a spread with a red and a green box
5. Select the two pages in the pages panel. Right-click and choose "Override all parent page items"
6. Move the green box from page 3 to page 2 by dragging it. Move the red box from page 2 to page 3.
7. Drag the parent Page A to page 3.
8. BOOM: the green box on page two is deleted. It should NOT be. The red box is kept. As it should.

If you instead of moving the box, use cut and paste it will NOT be deleted. [works as expected]
There are many ways to Override an item from its parent pages. All gives the same error.
It will happen on any number of objects or groups.

If an item is released from its master page it should not be affected by changes on the master page. I consider this a bug. This has been so for years.