Maybe the zero infinity point is just an abstract notion and therefore we are not allowed to calculate (-k*G + k*G) as a normal addition.
You can't calculate -P+P in a "normal" way, because this points have same X, so it will lead to division by zero (or, if we talking about EC math - multiplicative modular inverse of zero which doesn't exist).
-2*G + 2*G=(49667982834466148699028630885550314015746939561788444090107179747772288677390, 37758011528734597361759657276645959964664126776856991748971785837209648797521).
Nobody can't help you here, until you explain how exactly you're calculated this numbers