How could I guess? You didn’t even show full page and picture is blurred. The same image fragment is equally valid for paragraph, table or frame border.
Were your borders intentionally added to print on every page? Or are you asking for the non-printing clues which are only visible on screen (as layout guides)?
Latest update? We don’t know when you last updated. Releases proceed at their own pace on different platforms. Under my Fedora 43, this is today 25.8.2.2-580 Build 2. Tomorrow it may be different.
You save .docx which is not a LO-native format.
With an .odt, I’d have suggested to look at your page style. But DOCX has no page style. Consequently, every time you save, page styles are converted to the closest DOCX equivalent (and converted back when reloading). The roundtrip is not idempotent (meaning, you save a document and reopen it, the structural contents is not the same).