Originally Posted by
Points Scrounger
It was explained to me that it is one credit per night stayed, starting with check-in and ending with 4 p.m. the next day. So, if you use it the evening you check in, you aren't entitled to a credit towards breakfast the next morning.
That makes complete sense and is consistent with the "does not accrue" language in Marriott Bonvoy Ts&Cs.
OP said nothing about having already used the credit on the evening of check-in or whether it was one-night or multi-night stay. However, my impression was the OP had a one-night stay and was told the $10 USD F&B credit for member +1 could not be used for breakfast.