Partial gel
Updated
A darker translucent circle in the center of a bar with a paler opaque ring around it, which is a recognizable cosmetic fault in cold process soap.
A ring in the middle
The middle of the loaf got hot enough to gel and the edges did not.
Gelled soap is deeper in color and more translucent, so the two zones look like different soaps, with a visible boundary.
Every bar in the loaf shows it, which makes it a batch fault rather than an occasional one.
Why it happens
Heat builds in the center of a mass and escapes at the edges. If the center reaches gel temperature and the edges do not, you get both states in one loaf.
Which is why it is most common at the middle of the range: enough heat to gel partly and not enough to gel through.
A cool room, a mold that loses heat quickly, or a batch made slightly too cool are the usual circumstances.
Why it only matters visually
Both zones are fully saponified and perfectly good soap.
The bar will lather, last and behave normally.
It is a presentation fault, and it is a serious one commercially because it is so visible and so clearly inconsistent.
Both fixes
The rule is to commit to one state rather than landing between them.
Force full gel: insulate, soap warmer, use a mold that holds heat, or put the mold somewhere warm.
Prevent gel entirely: soap cooler, do not insulate, give the mold airflow, or refrigerate it.
Either produces a consistent bar. Doing neither reliably produces this.
And note that an individual cavity mold rarely gels at all, which is one of its advantages.