I had a similar case. FOB after refueling was what I ordered for a 3 hour flight.
During the climb phase, ECAM Caution came on for a F.Used/FOB Disagree - Fuel Leak Proc Apply. We lost about 1000kg. We went through the QRH and fuel leak source was inconclusive. We stopped at the part where we were asked to shut down engines as we were still 15 minutes away from the point of departure and fuel missing was down to 600kg. After 30 minutes, it became 400kg. So we decided to get to cruise level and monitor further. Short story we got back to FOB at departure after 10 minutes in cruise.
Not sure what happened there but it was my first time. Could it be water in the fuel?