How is the Jet pump able to maintain suction
The engine fuel supply runs from the tanks through the "spar valve" (LP fuel valve) located on the wing front spar. The fuel supply then runs "downhill" through the pylon to the engine driven fuel pumps. When you are "gravity feeding", the lowest pressure in the fuel run is at the highest point which is near the spar valve (during cruise or climb). Fuel is being delivered through the plumbing because the engine driven fuel pumps create a lower pressure on the "suck" side. Provided the fuel remains liquid in the plumbing system then the engine will run happily. If the pressure gets too low anywhere in the plumbing and vapour (gaseous fuel) forms then the "syphon is broken". The mechanical fuel pump can't pump vapour efficiently and the engine is starved. Hence the published gravity feed ceilings (where atmospheric pressure is needed to stop the fuel forming vapour anywhere between the mechanical fuel pump and the fuel tank inlet).
Since the fuel is being "sucked" from the tanks (really atmospheric pressure pushing the fuel through the plumbing towards the low pressure at the mechanical pump entry) and the jet pump in the centre tank works in a similar way (pressure differential i.e. suck) then apparently some of the centre tank fuel can be extracted whilst gravity feeding.
See engine driven fuel pump:
eaton fuel pump
Please see EASA warning Airworthiness Directive
AD No.: 2016-0205
Issued: 13 October 2016
google.co.uk.easa.europa.eu%2Fad%2F2016
"Reason:
Airbus introduced mod 154327 on A319 and A320 aeroplanes which substituted the pump fuel feed system from the centre fuel tank with a jet pump transfer system, based on the Airbus A321 design.
Subsequent investigation determined that ground wires had not been installed on the Fuel Level Sensing Control Units (FLSCU) of the modified A319 and A320 aeroplanes, due to a drawing error on the fuel system recirculation Principle Diagram. Without these ground wires providing inputs, the FLSCU logic is not correctly implemented for gravity feeding operation.
This condition, if not corrected, could lead to reduced fuel pressure at the engine inlet, possibly resulting in an uncommanded in-flight shut-down when flying at the gravity feed ceiling levels, as defined in the Aircraft Flight Manual (AFM)."