Sounds like the TA screwed up.
But did you ask for any flight change? It appears that nowadays most airlines will update the fare if you make any flight change on your itinireray. I had a paid F ticket HKG-JFK purchased before a 15% rise in fare last Autumn. I was specifically warned by tthe TA and CX that, although the ticket was fully flexible, any change of flight/day would imply a readjustment of my fare to the new one.