The AE algorithm evaluates the current image brightness and adjusts shutter and gain so that the image brightness is within a small range around the set value. In your usage scenario, white clouds will slowly drift by, and may occupy a significant area in the image. The brightness of the white clouds at night is much brighter than the sky, which causes the AE algorithm to evaluate the significant changes in the overall environmental brightness with the white clouds.
I feel that there is no way to keep this situation constant unless you use fully manual exposure. However, this will cause more serious problems. The camera loses its ability to adapt to the scene, so this is not recommended.