Is your characters capsule and camera moving up into the air with your character mesh? Or is the character mesh leaving the capsule and camera?
Usually you are not able to activate a jump with the default unreal engine jump while in the air, which leads me to believe you might be using a jump animation with root motion that takes longer than the actual jump. This would cause you to be able to jump again before the animation is finished playing, restarting the animation and gaining more height. Either that or you have a custom jump function that allows you to jump while isFalling is true