Selecting the "Document Modified Date" rename option uses the modified date metadata embedded in the PDF file in the renamed file name.
Win2PDF Auto-Rename Document Modified Date
Some documents do not have the "Document Modified Date" metadata set in the PDF. If the "Document Modified Date" metadata is empty, a "-" character is substituted in the renamed file.
You can view the PDF metadata fields by opening the PDF in Adobe Acrobat Reader DC and viewing the "Document Properties" under the "File" menu.