A centered circle or uneven brightness in astro images is often caused by lens vignetting, and the EF-S 55-250mm is known to show noticeable vignetting. It can also be made worse or altered by automatic lens-correction settings during processing.

Things to check:

• Disable any automatic lens corrections and compare results.
• Make sure your calibration/workflow is handling vignetting consistently.
• Test at different focal lengths and apertures to see if the pattern changes.

If the effect is strongest toward the edges and leaves the center brighter, that points to vignetting rather than a sensor problem. Since it appeared recently, it’s worth checking whether any software setting changed in your processing pipeline.