First off chill out. You didn’t get a masterworked drop. If your inventory is full and you try to pick up a drop, I’m sure you see the little masterwork effect animate across the weapon along with the big red X that let’s you know your inventory is full. This does not mean the weapon is masterworked. This means the animation is glitched to sometimes play when your inventory is full. So when it goes to your postmaster, it’s not “losing its masterwork” because it never was masterworked. If you deleted something in your inventory then picked it up it would be a regular weapon.