Phraseology
I note with interest the phrasing of the AAIB update:
Begins:
Under the direction of the AAIB, extensive full scale engine testing has been conducted at Rolls-Royce, Derby, and fuel system testing is ongoing at Boeing, Seattle.
Ends.
So, to me that suggests that whatever has been done at Derby now is complete, and that the work at Seattle is continuing.
I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason.