Add child folder does not remove the folder from the root folders

1. Create a level sequence
2. In python:
   sequence = unreal.LevelSequenceEditorBlueprintLibrary.get_focused_level_sequence()
   folder = sequence.add_root_folder_to_sequence('Folder')
   child_folder = sequence.add_root_folder_to_sequence('ChildFolder')
   folder.add_child_folder(child_folder)
3. Save/Close/Reopen the level sequence

EXPECTED: ChildFolder should be a child of Folder.