Unless there is some operational reason (possible pending aircraft swap, "inoperative" seat close to departure time, "government business," etc.) to restrict F, I've never otherwise seen it happen personally. They have every incentive to keep that F space available for passengers willing to pay First Class fares (or peak First Class Mileage Plan awards). Previous posters are correct in their thinking that with all the AS and OAL reservations agents, travel agents, alaskaair.com, online travel agencies, cruise lines, etc., etc. it is not unusual to see such fluctuations within the same day, particularly when you get down to those last few seats!