Just brainstorming here....

Could this grey stuff be the original base of the bead?
I know some beads used to me made with a different core. Considering the fact that the bead is ground on both sides, that may be what revealed the inside. Seems to me like the most logical way to get it in the middle.

In Hans' Flick Album ' Boulevard of Broken Beads' there are a few beads with a similar core. Akoso and faience. Don't think that is what this bead is, though. I think yours does in some way look reworked. Not just the grinding, but also the swirls.
It is a fascinating bead, anyway.

Hi Rosanna,
I don't think, this bead was repaired, I would think rather that it came during the making of the bead. The both sides of the bead were exposed differently on the fire. It would be the ground of the bead which melt down a little bit differently because the heat and the temperature of the melting were different, maybe a bit less. There are two pics which are less clear than yours but we can see that the both sides are different, and watch a little the same difference as yours.
Sincerely,
Kika

The other bead

I'll bring this bead to Tucson for show & tell.

Hi Rosanne,
After searching I could not find any bead in my Krobo collection what resemble yours.
The outside looks like a buffed Venetian bead to me.
Is it possible the perforation of the bead is narrowed in Ghana with powder glass and maybe cracked after the reheating?
A nice time in Tucson, Hans