[2019] Staff of Darkness durability not decreasing?

Sorry if this has been asked before.

So I made my Staff of Darkness about 2 weeks ago and took it to numerous slayer tasks already, even fight kiln and some godwars, though the Item Charge remains on 100%.

Is this some sort of visual bug, cause I'm pretty sure it should be down to at least 70% by now, or does the item charge only change once it reaches 50%?