Why does the equation $v^2 = u^2 - 2gh$ give wrong height when a cannon ball is fired at 10 km/s straight upwards?

Question

Accepted Answer

because $g$ is not constant; it decreases far from the earth's surface

Answer

because air resistance is ignored

Answer

because the ball is too heavy

Answer

because the equation is only valid for horizontal motion

Why does the equation v2=u2−2ghv^2 = u^2 - 2ghv2=u2−2gh give wrong height when a cannon ball is fired at 10 km/s straight upwards?