Will attack rating of my sword decrease when leveling up?

I just got a new weapon (Watcher's Greatsword), upgraded it to +5 and infused it with magic.

Now, I wanted to spend my souls on leveling but when I pull up the leveling screen and change the points of an attribute, the attack rating of my main weapon (said greatsword) drops drastically.

Is this really the attack rating the sword will have after my level-up? The weapon would become worthless for me. I can only assume this is a bug. I'm stuck with a lot of souls now because I don't want my weapon to loose so much power.

In the top right corner it displays a drop from 487 to 327.

I also un-equipped the weapon and used another one instead. For other weapons I don't see any decrease. Why would there be any? But, for this one greatsword it shows this huge drop in attack rating.

Anyone got a clue what is going on here? Can I safely level-up with loosing that much attack rating on my greatsword?