As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT CREW REPORTED THAT WHEN THE PLANE SAT OVERNIGHT, THE CENTER TANK WOULD HAVE MORE FUEL IN THE MORNING. WHEN THE NEW WING ROOT FLAPPER VALVES WERE INSTALLED THE FUEL MIGRATION CEASED. THE OLD (REMOVED) FLAPPER VALVES WERE MISSING THEIR RETENTION SPRINGS ALLOWING FOR A BAD SEAL. CAUSE: NO RETENTION SPRINGS ON THE FLAPPER VALVES.