Another possibility is that back in February, there might have been married segment logic applied where an "F" leg cannot be combined with a "J" leg. (eg. Availability disappears on the "J" leg once an "F" leg preceding it is booked).

This is one of the revenue management tactics airlines use. Sometimes if there is a fare that is "low" or "good", they manipulate their inventory (during the booking process for a particular "work area") to try and prevent people from booking it.

As time progresses and they have trouble selling the seats, they may lift the "married segment logic" for a particular city pair to allow the lower fare to be booked.