Any custom data type for editor alias Umbraco.BlockGrid if defined not in main usync folder/root -- after each re-serialize appears in root, despite being already defined in another folder.
It basically produces duplicate of exact same configuration for some reason. Only applicable to data type, elements from the very same secondary usync root are kept in secondary root and they are not re-created in main usync folder/root.
(I hope it makes sense, a bit weird wording)