Often custom post types are defined with the same capability type – default ‘post’. That is they all use the same capabilities list: ‘edit_posts’, ‘delete_posts’, ‘publish_posts’, etc.
When you turn ON the checkbox for ‘edit_posts’ which you see under some custom post type group, you grant this capability not to this custom post type only, but to the currently selected role. Thus this role can ‘edit_posts’ for all custom post types including WordPress built-in ‘posts’. That’s why you see it is “applied to all other custom post types’.

So its not a bug, its a feature.