There is a supplementary procedure FMS specificities named "ERRONEOUS ESTIMATED FUEL ON BOARD (EFOB) WHEN THE FUEL FLOW ONLY (FF) IS USED"
According to which :
FOB is the computed value of the Fuel On Board. It is based on BLOCK entered by the flight crew on the MCDU and : ‐ Data received from Fuel Flow (FF) and Fuel Quantity (FQ) sensors (from Fuel Quantity Indicator System, FQIS), or ‐ Data received from Fuel Flow only (FF), or ‐ Fuel Quantity (FQ) only With the FMS REV 2+ and previous Thales' standards, an anomaly has been reported on EFOB computation in case of FQIS failure, or when the flight crew selects the FF only. In cruise, when the FF only is used by the FMS and is below 2.7 t/h, the FOB becomes frozen for predictions. Therefore, the EFOB constantly increases at all downpath waypoints and the FOB value at the FROM waypoint displayed on the F-PLN B page is frozen. The FMS uses the FOB to compute the Estimated Fuel On Board (EFOB), at each waypoint and at destination. The EFOB is displayed on the MCDU on the page F-PLN A and FUEL PRED. However, the FOB displayed on FUEL PRED page is correctly updated (based on FF only). A manual re-entry of the FOB through the MCDU enables to successfully update the FOB used for predictions, including the EFOB.
So yes it's a known issue
