Could the drag and drop not be working in the new pages MyImageGalleryPage because of this that I read in the docs?:
"Activate Drag-and-Drop Sorting
Because drag-and-drop sorting adds custom functionality to all eligible DataObject subclasses, the DataObjectManager requires that the user opt into sorting capability.
mysite/_config.php
SortableDataObject::add_sortable_class('Testimonial');
After running a /dev/build, the following will be modified to give Testimonials sorting capability.
*
The field SortOrder will be added to the Testimonial table
*
The $default_sort property of the Testimonial will be set to SortOrder. This will ensure that the frontend template displays the objects in the correct order.
Note: To add several sortable classes at once, use SortableDataObject::add_sortable_classes(array $classNames)
"