as mentioned in T93525#2404697, attempt to edit header/footer break them both (moving them or their parts into main edit area, header disappeared and been replaced with footer content, footer have been transferred and mixed with the contents of the page...)
It is easy to reproduce:
- step: Editing header and footer (added <div style="color:blue;"> in header and </div> in footer- see image
- step: Save Page -> Review your changes see image
This is not unusual edition - such a way of edits (use) of header/footer in the Proofread is common. They are used as parts of the page (before and after content) which may not be transcluded to the main namespace (noinclude) - and often used to set a style different from the main ns.
See: thousands of pages in fr ws
thousands of pages in ef ws
thousands of pages in pl ws
Changing the way of functioning headers/footers would require changes in many thousands of pages on ws and also editing habits of users.