Maybe try replacing the code above with:
add_action( 'init', function() {
add_filter( 'rwmb_meta_boxes', 'prefix_register_meta_boxes' );
}, 5 );
And check the code to register terms, make sure it's hooked into init
with priority 0, like this:
add_action( 'init', 'prefix_register_tax', 0 );