I have never seen this
I have added a Blank CSS.
Then copied an inline (same book) CSS CODE into the new CSS,
Then Linked the CSS to the file.
Then used the original source Includeing the <style> to S&R (normal) to remove all.
Saved just fine (later edit)


I have seen a couple of mentions of F8 Metadata edit.
That is one step I have not used in this type of CSS situation