Sumping - easier said than done sometimes...
No water in the fuel at cruise - too cold, it is all ice.
Descent to a surface still below freezing - atmospheric ingestion contains water vapour which at lower altitudes condenses as frost on the cold tank surface.
If ambient stays below freezing, frost stays on the tank walls. Nothing to sump.
On the return journey, same story except that at lower levels the ambient is above freezing and the frost melts.
The melt water from the centre tank is sent into the still cold fuel in the two main tanks where it freezes again...