What you’re seeing is usually tonal banding/quantization, not true pixel size issues. When you darken a smooth sky, Lightroom is spreading a small number of similar tone levels over a larger visible range, which makes differences between adjacent tones easier to see. Smooth areas like skies reveal this first.

It’s more likely with JPEGs because they contain less tonal information than RAW files. A stitched panorama may also give you less headroom than editing the original RAW files first.

Ways to reduce it:

• Prefer RAW/DNG source files over JPEG.
• Avoid heavy darkening of already smooth sky tones.
• Make sky/exposure adjustments on the original files before stitching when possible.
• Use selective smoothing/noise reduction in the sky to hide banding while preserving detail elsewhere.
• If the scene exceeds the camera’s dynamic range, capture more tonal range up front using exposure blending/HDR.

In short: the sky is banding because the edit is stretching limited tonal data, and the best fixes are more source data, gentler edits, or smoothing in the sky.