Updating a custom post type post using rwmb_set_meta with a custom table

When using rwmb_set_meta, it updates the meta value, but it is not saving the value (so the old value is still in the DB).

Based on Meta Box documentation, although it is really unclear re. the add_action, it suggested that this would save the value.

I have tried this (with the correct values and also by manually adding test values):

1) rwmb_set_meta( $object_id, $field_id, $value, $args = [] );

and also:

2) add_action( 'init', function() {
$post_id = 123;
$field_id = 'my_field';
$value = 'My custom value';
rwmb_set_meta( $post_id, $field_id, $value );
}, 99 );

Neither updated the db. 1) did update the object until page refresh, 2) did nothing.

Question is, what do I need to do to update the Meta Box value in the db?

Again, the documentation really needs to be updated here. I think it would be great to get some of the information in this forum in the documentation.