Read the OEB and you will see. There is full page.
On a certain type of pump screw was found loose.
This "cause the pump's internal flame trap to be uncovered".
Under such scenario, the separation of pump's electrical components from fuel-air mixture cannot be guaranteed.
Adopt the procedure to run the pumps only fully covered in fuel. All pumps off for refuelling. Center pumps of when less than 2t.
If you do not read the OEB, or you are not provided with it, it is hardly Airbus fault.
FD.
(the un-real)
To say that center tank fuel below 2 tons is unusable is simplification.