Damage calculation Bug

So I restarted the game after the full release and have encountered a weird bug with my Fighter. When he gets a proc on Great Weapon Fighting he rolls a d4 for damage instead of his great axe's d12 damage die. I think it might be because he has the "Follow up Attack" feat which allows the character to make an attack with its bonus action with two handed weapons using a d4 for it's damage dice. I've only ever seen it happen on the second attack on his turn.