It happens with the small and large cumulus clouds if you turn on the option of rendering the clouds as fog.
Seems like the fog patches don't match the real volumetric clouds and they appear randomly instead. Also, in some places you get clouds appearing from nowhere

(the brighter clouds are the ones rendered as fog)

the render clouds as fog setting has fallen behind on my changes to the actual clouds. as a result of that, their lighting and shapes are no longer matching. disable this setting until i fix it up