[Edit] It seems like it's an issue with Tinkers Construct Weapons

The black/gray underlay constricts to the item with the lowest durability image. As seen in the image the sword has the lowest durability so it contracts to it, but when I unequip the sword it goes back to normal and works fine since they all have 3 digit durability.

How To Fix: I am not sure if this is possible, but I would think would work is to first make it constrict to the largest digit durability rather than the smallest, and then center the other durabilities in that length.

I was under the assumption I already fixed this. Appears I was mistaken. I'll fix it.

I am not sure if you saw my edit or not, but it is only a problem with Tinkers Construct Weapons.

There was an issue with the length calculation. Should be fixed since 3.6.9