Covers block uses 'post-thumbnail' image size for background images.
It doesn't seem that images stored in media library have the specific image size/thumbnail generated.
https://github.com/greenpeace/planet4-plugin-blocks/blob/master/classes/controller/blocks/class-covers-controllers.php#L160

Check gpi act page loaded images.
https://www.greenpeace.org/international/act/

Find and use a better suited image size for the block's background image.