Created attachment 205102 [details] DOCX When saving DOCX, the visual gap in the horizontal line is missing so line appears single. MSO uses text box in front of text to give impression of two lines. LO opens that text box with wrap in background. Similarly it is behind with saved DOCX opened in MSO.
Created attachment 205103 [details] DOCX compared DOCX compared when original and saved opened in MSO.
Are you sure the attached DOCX is right?
Yes. Opening in MSO should show line in the header split with the gap, as shape is above the line. In LO it appears as uninterrupted line as shape is below.