Could be an internal reflection. The optical path for many JWST instruments is, uh, compact: https://www.esa.int/ESA_Multimedia/Videos/2021/09/Webb_MIRI_... (Every kilo on telescope structure you save by folding the optical path is a kilo you can add in propellant, extending the working life of the spacecraft)
Instrument internals are painted black and heavily baffled, but nothing in optics is perfect. Dithering the direction the telescope is pointed in and image stacking should cancel out most optical artifacts, but internal reflections will be worse for bright objects like stars, which JWST probably isn't usually going to be observing with the imaging instruments.
Instrument internals are painted black and heavily baffled, but nothing in optics is perfect. Dithering the direction the telescope is pointed in and image stacking should cancel out most optical artifacts, but internal reflections will be worse for bright objects like stars, which JWST probably isn't usually going to be observing with the imaging instruments.