Trying to book a flight from EWR-SEA next week, and when I'm trying to book the return for Friday, one of the options (my preferred option) is only showing first-class availability, even though there appear to be a number of empty seats in economy (20+ seats). I know the fare class availability and seat map don't always line up, but this seems to be a pretty huge discrepancy.

Has something like this ever happened to anyone else? Is it more likely that the seat map is wrong, or the fare class availability? I'd really like to book that flight, but client won't go for a first-class ticket.