Same thing happened to me. You do get two perks points when you hit 275, choose legendary smith and you still have one, then after you go up more in smithing you have the siege espert on top of legendary smith (which you can't see anymore) and can select it. Now you are no longer showing an available perk point. I wonder if they are both active, if it's a visual bug.
I started my game in 1.5.0.