I came across something similar the other day when I was looking to book an award flight. I briefly selected the flight and moved to the next screen before second guessing myself and leaving the booking process to think. When I searched again it showed the seat was gone, in a similar fashion perhaps to what you were seeing (the column was there but there was no availability – whereas usually when there's no availability the column isn't even visible). I later worked out that it was holding the booking for me for about 10 minutes, at which point a subsequent search showed availability again. So perhaps someone else had temporarily held the seat while they were going through a booking process, and if they subsequently don't book it, it'll re-appear?