I'm not sure if this bug works as intended or not.

In the history of minecraft, there have been different generators of "floating" or "sky" worlds at different times, well, concretely two (the third one is the actual).
The first was meant for world customization and the second for a new dimension.
No long time ago I discovered that both of them shared a feature: the clouds are under islands. Personally, I think it fits very well with the "sky" theme.
Later, I discovered something else: the class net.minecraft.world.level.dimension.end.TheEndDimension has a method (getCloudHeight) wich says that in this dimension, if clouds were enabled, they would render under the islands (Y 8).
This method is a remainder of the code of the unused "Sky Dimension".
This method also appears on its abstract class net.minecraft.world.level.dimension.Dimension, but it says the clouds are in Y 128, so I think this method is still used by the rendering system.
Today I looked if this method stills on the code using the obfuscation maps, and yes, it's still there.
The actual Floating Islands world type is based in the chunk generator of The End (because it was called "End" before 18w19a), wich it haves this hidden feature.
I believe that minecraft team forgot to add this when they were working on the actual Floating Islands world type.
I'm really not sure if you will take it as a suggestion or a bug. I think it's a bug, but i'm not sure.

Note: This method makes the clouds render under islands ONLY in Floating Islands world type.

//The method I writted below is exactly how it would look ifs following the obfuscation mappings of 20w21a (https://launcher.mojang.com/v1/objects/a41221343ccf3bd58dec5b5fc091ce98b27003cb/client.txt)

//So you will only need to go to "net.minecraft.clien.renderer.DimensionSpecialEffects." and paste this method

public final float getCloudHeight()
{
    if (this.level.getLevelData().getGeneratorType() == net.minecraft.world.level.LevelType.BUFFET)
    {
        if (this.level.getLevelData().getGeneratorOptions().getCompound("chunk_generator").getString("type").equals("minecraft:floating_islands"))
        {
            return 8.0F;
        }
    }

return 128.0F;
}