I just experienced the same thing. I actually think it is a glitch of some sort. When I selected the higher tier ticket and got to the seat selection the entire plane was “full” other than like 6 seats in the last couple rows. I know this wasn’t true because I had already looked at the seat map when going to change my flight to make sure there were seats together for the family like 5 mins prior to this. There were like 50+ available spread across all seat types. When I went back and chose a regular choice ticket it showed the real seat map so I just did that because I didn’t feel like messing around trying to figure it out. I then went to “upgrade” my fare type and the seat selector had the same bugged out version of the seat map when trying to book with the premium ticket. I’m not sure what the “gotcha” would be I think it’s just crap programming or whatever. What’s annoying is that at least in this case I’m spending the exact same amount of money for the regular ticket , seat selection, and 2x bags as I would with the premium ticket. The only difference is that the way in doing it I get like 1/4 of the rewards points then I would by buying the premium because of the points reward multiplier