This might not be a bug, it is possible that if a higher fuel load results in the C o fG moving back then this would have a beneficial effect, a C of G which is further back needs forward elevator to maintain level, the side effect of this is to get some extra speed in level flight through a reduction in drag caused by the elevator, this is an effect some modern passenger jets use where they deliberately pump fuel to tanks at the rear (in the tailpane itself) for that very reason. concorde is a famous example of a 'fuel trimming system' but for slightly different reasons.

Note: I say 'might'.....I have no idea of spitfire load and balance.

Interesting. A WW2 nightfighter ace, Martin Drewes, used to remove the camouflage of his 110 and was capable to fly some km/h faster.

I am with you on this.

The fuel tank is also closest to the CG on the 109.

That was not the weight effect, it was a perfect filled and polished finish of the outer skin to reduce drag...