Oh, sounds not fun at all.
CX booking engine can display flights available even when no seats are left in service class, with a small marker and a footnote saying "booked in class x, charged as y"
In my experience, AY booking engine does not do this, instead displays no flights available in such a situation.
So if there is a change in how AY booking is made, or if there is another explanation to what happened, it would be interesting to hear.