my embassy (Finland) also states they dont give a reason, but yet for me, they have told me both times i got rejected. So yes, its possible they will tell you.
no, i did the first 90d report on day 86, so the next due date was 4 days before the date on my stamp, on which i did border run, and forgot the 2nd 90d. And after that i did my 90d report on day90, no problems. Jomtien IO