Hi Superpilot,
About 6 years ago there was notice about the center tank pumps running during refueling despite the tanks being empty (dry sensor). The pumps would then become "un primed" which meant they were unable scavenge the center tank dry. This eventually resulted in carrying about 200 kgs of fuel in the center tank which was unuseable. (If the center tank was then fueled with 500 kgs, the pumps would "prime" and all the fuel could be scavenged).
Until the new logic for center tank pumps is fitted, we still turn the center tank pumps off during refueling.