I’ve actually spoken the Elementor devs about this.
They’re using the excerpt_length filter, which is a global filter inside WordPress.
Instead, they should be building their own excerpt function as I did in WP Show Posts.
Right now, the GP excerpt length overwrites the Elementor length, as it should in my opinion. If it was the other way around, whatever length you have set in Elementor would apply to the GP blog, archives, categories etc..
You can tell GP to take a back seat to Elementor like this:
add_action( 'after_setup_theme','tu_change_excerpt_length' );
function tu_change_excerpt_length() {
remove_filter( 'excerpt_length', 'generate_excerpt_length', 999 );
remove_filter( 'excerpt_length', 'generate_excerpt_length', 20 );
add_filter( 'excerpt_length', 'generate_excerpt_length', 15 );
}