I never paid attention to this before I started flying the 135 in the military version.. (self sealing tanks).
There this is a real problem....
Later on I learnt that this seems to be specific to bulletproof ("self-sealing") tanks, and on the BK117 we could actually solve this by opening the floor panels and readjust the fuel tanks..
Believe it or not.. It worked..
"Stretching" does not mean then material is giving in... But the tanks in some helicopters seem to need some time until they can be filled to max..