This seems to happen to most of my bookings made lately, but only after the initial seat selection. Usually with the message "This reservation cannot be modified online as one of the flights in your itinerary may not be eligible for changes."

Could it possibly be a Jazz thing? Or something much less tinfoil-hat-suspicious?

Either way, it at least lets me select a relatively nice seat as far up as possible but it had better be a good first choice.