NEVER allowed to be soft soldiered. There is a provision to allow for
brazing but you must use a special rod, which I have never been able to find
in over 20 years.
ONLY flare nuts with copper are allowed, compression isn't allowed either.
You are sitting on a bomb just waiting to go off. Fossil fuel gas attacks
copper and causes flaking which can cause a gas valve to stick open. Also,
since it flakes it attacks the copper itself which means that it attacks the
copper that was soldiered, which means the joint constanly get weaker by the
second until the joint fails. With a flare it is less likely to have such a
major leak since it is a mechanical joint, not dependant on a joint like
being 'glued' together.
Change it out, use Stainless steel. Better yet, since you dont know the
codes I would stick to painting and call a plumber to put it in correctly