I'm referring to the fact that bribing increases your speechcraft skill and not your mercantile skill.

I wouldn't really consider that a bug? Not sure if it could be changed in the CS either, if not, you'd have to either wait for OpenMw or see if Hrnchamd will do it.

It is a well known bug, and I don't think there is a fix. (I'm surprised the Code Patch has never touched on this, unless it's something unfixable in the code we are not aware of)

EDIT: Doing some searching, It seems I actually asked this very question in the MCP thread two years ago. Hrnchamd never responded to it, which either means it can't be fixed, or he needs to be reminded

But, your Bribery success is still based off of your Mercantile skill, despite being influenced by Speechcraft.
So the mechanics still supposedly work, if you know that loophole.

Honestly, I think it should be based from (and raised with) your Speechcraft and not your Mercantile.
I am thinking this bug exists because the devs just messed up the code somewhere, from a miscommunication about which Skill would influence Bribes.

Agreed. Bribing isn't haggling, it's speech influenced, it's not about spicing up an offer.