BuddyPress.org

I’ve duplicated the issue for plugins using code like this:

add_action( 'bp_include', function() {
        if ( class_exists( 'BP_Group_Extension' ) ) {
                class My_Group_Extension extends BP_Group_Extension {
                }

                bp_register_group_extension( 'My_Group_Extension' );
        }
}, 99 );

The BuddyPress Cover Photo plugin uses something similar to the above.

I’ve opened a ticket with a fix to this specific problem:
https://buddypress.trac.wordpress.org/ticket/7140