Hmm, it might relates with the way you register meta boxes. In 4.9.x, the biggest change is the WPML compatibility, which **forces** Meta Box to use init action to register meta boxes instead of previous admin_init.
So, if you wrap your code like this:
add_action( 'init', 'your_function' );
function your_function() {
add_filter( 'rwmb_meta_boxes', 'slug_register_meta_boxes' );
}
where init or any other hook is used, please change it to:
add_filter( 'rwmb_meta_boxes', 'slug_register_meta_boxes' );
I'm not sure if that's the correct problem, but some people meet it, so I guess you do. Please check your code for that.
.
