Ah, that's interesting.. My theory is, in those rarer cases like mine, something happens -- like someone booking a last minute revenue ticket, or a higher status individual changing to an earlier flight -- that consumes the F inventory that hasn't been properly locked/held. On this theory, I guess my approach next time will be to try to get a seat assignment right away from ticketing or an agent, minimize the window of opportunity to lose the seat..
Poked Alaska Cares with a gracious note and will see if anything comes of it.. *shrug*
thanks!