The most likely cause is software correction, not a lens defect. In very dark scenes, phones often apply lens-shading/vignetting compensation that brightens the corners relative to the center. When you push exposure in a RAW night image, that compensation can make noise and tonal differences appear as circular rings.

One answer suggested internal reflections, and that’s possible in some setups, but the ring-shaped increase in grain toward the outer areas is more consistent with digital correction being amplified by heavy editing.

Can it be fixed? Usually not completely from a single frame. You may be able to reduce its visibility with careful local adjustments, noise reduction, and by avoiding extreme exposure lifts. Additional images can help only indirectly: stacking multiple exposures can improve signal-to-noise and make the artifact less obvious. In normal brighter scenes, this effect is often much less noticeable.

So: it’s probably a low-light processing/noise issue revealed by strong brightening, rather than something physically wrong with the lens.