In Writer, it seems that disabling “Apply user data” in File Properties will cause the “Modified” field (also in File Properties) to always be blank when saving the document. Is that intentional? It’s still possible to see when the file was modified through File Explorer, so I’m not sure if that’s a bug or if it’s on purpose.
Tested on 7.3.7.2 on Linux 6.0
I mainly work without user data and the Modified field always returns expected time and date. Whether Use data for document properties is ticked or not makes no difference.
Do you save .odt?
Here’s one that I made for the purpose of testing this. I’ve saved it multiple times, but no modified field.
Untitled 1.odt (9.8 KB)
Probably a former DOC(X) file.
The Inspector shows “Wordmodus”.
I am not familiar with this term in Writer.
Paragraph and character entries without end.
If not DOC(X), then formatting hell. 
With a new empty file and change there are no problems.
IMHO, doesn’t seem to be a former DOCX. However, adding modified information result in void field.
I saved as .fodt and looked at the XML. There is no meta information about the edit history of the file. This may be the cause of the void fields. Perhaps the file was damaged during a session and LO can’t recover from it.
FTR: tdf#152578