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I think the problem is banding caused by not having enough colours. The first image only has 163 colours.

How are the images being created? Is the original a gif (which has limited colours)?

I tried (Filters > Blur > Gaussian Blur) with a value of 12. The number of colours in increased from 163 to 53315. The banding is reduced.

I don't see anything abnormal here; This is 8-bit per channel imagery. There are round-of errors and limitations. So yes, there is is banding in your cloud/sky picture but you really have to put your nose on it to see it. You can mitigate it using Filter>Noise>Spread.You can also use Gimp 2.9.x and do the image in 16 or 32-bit mode (but look out for surprises when you export the final result, because that will be 8-bit...).