Can you please show us a screenshot to see how you use this type of Action?
Have you checked Alt + T to see if multiple fields with the same name have been created?
BTW: The Action "Split field by separator" does not work for _FILENAME (included in _ALL), because it only splits the content of tag fields by separator, not a filename (nor a directory name).
Special Fields
_TAGcovers all tag fields_FILENAMEdenotes the file name of the file_DIRECTORYdenotes the parent directory of the file_ALLcontains both_TAGand_FILENAME(not_DIRECTORY)