I did exactly as you instructed, although that meant making no changes since "Do not check spelling or grammar" was already clear. When I ran the spelling and grammar check again, the problem persists although, strangely, this time spelling errors in my comments were detected. It may be relevant to know that even though I selected Ctrl-A before running the spelling and grammar check, I got a message like "Do you want to check the rest of the document?". I responded in the affirmative. The same problem appear in other documents (even though I reported otherwise before): even though the Ctrl-A does select the entire document, I get the behaviour reported in my first post: all readability metrics = 0 except for the first 3. And these are all wrong (in the sense of being way too low).