Custom blocks with alpha or blend materials will render behind the cloud layer when viewed from a distance.

Can be reproduced by adding a custom block and adding:

"minecraft:material_instances": {
    "*": 

{         "texture": "your_texture",         "render_method": "blend"     }

}
 
or,
 
"minecraft:material_instances": {
    "*": 

{         "texture": "your_texture",         "render_method": "alpha"     }

}