Some bookings don't allow the selection of seats prior to check in (e.g. Economy Classic). The seat map might look different based on that.
I've just read the website and it's not clear whether you can select a seat at all if you have an Economy Classic booking, even at check in time. 😕
If you are travelling in Economy Comfort or Economy Convenience, you can select a Standard Seat from the moment you book your flight. If you are travelling in Economy Classic, you can select a Standard Seat for a fee.
https://www.qatarairways.com/en-ro/c...rred-seat.html
Either way, once check in is open, the seat map that you are presented is the source of truth and it probably doesn't matter if a seat shows as available for a dummy booking.
The best way to get the seat(s) that you want? Reserve it before check in (you may have to pay for it, depending on the fare and your status).