I just read a piece of writing from our member Scarbs on Motorsport.tech. https://motorsport.tech/formula-1/fuel- ... it-be-done
It states that in 2019 teams could store up to 2 liters of fuel outside the fueltank (and therefor after the fuel flow meter). Could it be so simple that they have a bit of flexible storage between the tank and the high pressure pump to inject a few extra grams when needed? A simple air filled rubber ball inside a small acumilate tank would do that.