The most likely cause is inconsistent or overly aggressive pre-processing before the files go into Starry Landscape Stacker.

Based on the community replies, two common triggers are:

• Auto white balance changing slightly between frames
• Initial RAW processing adding contrast, sharpening, noise reduction, or lens corrections that don’t match what SLS expects

A safer workflow is to prepare all images consistently before stacking:

• use a custom/manual white balance, not auto WB
• reduce contrast
• turn off sharpening
• turn off noise reduction
• apply chromatic aberration correction only
• leave other lens corrections off

If one or more frames were processed differently, the stack can produce strange ridges, rings, or mottled artifacts. Re-export the source files with identical settings and the minimal adjustments above, then stack again.