Try this :
Open an existing odt document, then in the menu go to Properties, Font, and select "Embed fonts in the document" and press ok.
See that the Save icon is not lightened, then close the document (see that no warning is made).
Open again the document : fonts are not embedded!

For the setting to be taken into account, here is what the user has to do :
Open an existing odt document, then in the menu go to Properties, Font, and select "Embed fonts in the document" and press ok.
Change something in the text and immediately revert the change (since you don't want to change anything in the text : you just want fonts to be embedded!).
Now that the Save icon is lightened, press it and close the document.
This is odd, user should not have to work around that.

Thanks