A dark inner ring inside circular bokeh highlights is not the usual “bright-edge” (nisen) bokeh, and it also doesn’t look like classic “onion ring” bokeh. Based on the answers, the most likely cause is an internal lens element or element boundary affecting how out-of-focus light is rendered.

That can simply be a characteristic of the lens design, but it could also point to a defect, such as imperfect bonding/alignment of elements. The supplied hood is unlikely to be the cause, especially since this pattern is inside the blur discs rather than general vignetting or clipping.

Chromatic aberration exists in all lenses, but by itself it doesn’t best explain a distinct dark internal ring with a brighter center.

So the practical takeaway is: this is most likely an optical rendering trait related to the lens internals; to tell whether it’s normal or a fault, compare your lens against another copy of the same model under similar conditions. If another copy shows the same pattern, it’s probably normal for that lens design. If not, your lens may have an issue.