I don't wear an arm guard because I usually don't hit my forearm with the string. Like you, it sometimes happens, and it isn't always with a low brace height longbow. It seems to happen more with some bows than others, and I’m not sure why. Some bows have a grip that feels better when the bowhand is rotated more into the bow than is optimal, and that may cause string burn. Now that I know that, I tend to either stay away from those bows, or if it's a bow I already have, to reshape the grip so it feels more comfortable when held with the pressure point between the thumb and lifeline.