Why does LibreOffice-Writer change formatting and fonts, and remove headings and page numbers when I save a document?

I have a new Raspberry Pi computer with LibreOffice software installed.
I created a new document on LibreOffice-Writer using the supplied ‘Bitstream Vera Sans Mono’ 12 point font, but when I try to save the document I get a notification that says
“Confirm File Format
This document may contain formatting or content that cannot be save in the currently selected file format “Text”.
Use the default ODF file format to be sure that the document is saved correctly.”
However, if I do so, I lose the entire document, all I get is a blank page.
If I select “Text”, as predicted all the formatting is lost, as are the font, headings and page numbers. The font becomes ‘Liberation Mono’, the font size becomes 10 point, the line spacing, paragraph spacing change, the centring and right margin are lost, as too, italics and bold print selections.
How do I overcome this problem?

However, if I do so, I lose the entire document, all I get is a blank page.

This is strange, everything else is normal. Please describe this piece in more detail.

… and share a sample file.

OK. I think I’ve discovered my error by myself. What I thought was a blank document was actually the software asking me for a document name in order to save it. Once I gave it a name, it worked properly. Thank you for taking the time to reply.