Trap throwing speed bench craft on Belt

I just bought a Stygian vise with an open suffix and was planning to craft "Trap throwing speed" on it, but the game said the item already has a mod of this type (the one from the essence) and didn't let me.
But just last week I crafted the same thing on a Heavy Belt.
Is this a bug or they ninja change it?
This upsets me a bit cause I just wasted 5ex buying the Stygian Vise. :/
https://imgur.com/a/RNBdx9A: the Stygian Vise
https://imgur.com/a/xbWgkLa: the Heavy Belt

You have trap throwing speed on there already.