Qantas complaint: Seat Selection removed without communication

Good day, a few months ago after I purchased my flights directly from qantas and as a frequent flyer I was allowed to select my preferred seating on the qantas app. Yesterday I noticed that my preferred seating on my return flights was removed without any notification/communication and now all the aisle seats are allocated and unavailable.
The only seats available is one window seat with no window or a middle seat, I get claustrophobic quite quickly and thus pre-selects aisle seats.
I called the customer careline to enquire and they said someone must have paid for that seat and now its theirs. I’m quite upset and disappointed that Qantas let this happen and no communication on I have to choose a new seat. Has the no additional cost seat selection become a bidding war whoever pays more gets their preferred seat in economy, even though most seats are allocated without additional costs?
What I do not understand is if you would like to purchase a specific seat (extra legroom etc) qantas only show the seats that are unallocated and available to be purchased. I would really like to understand how someone could purchase an already allocated seat and how you can just bump off one customer for another.