Sprocket probably has it right.
There is an article somewhere in an Australian Flight safety magazine about how a Piper had what might be a similar problem.
In that case the fuel tank caps were replaced with identical ones that were non vented instead of vented. (model changes Cherokee or Warrior or some such).
So aircraft sucks fuel out of bags and collapses them because there are no vents, pulling everything out of alignment.
Next flight the pilot topped tanks up to brim and dipped them - everything showing full tanks - and then ran out of fuel two or three hours later.