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Peter,

I got it figured out. The custom field group I am using is for a Post Type, not a Taxonomy, so I believe that term ID doesn't actually come into play. This was obviously my misunderstanding.

Here is the working snippet if it helps anyone in the future. I simply needed to add the post ID to my initial code, where post ID is 123 in this example:

function shortcode_function(){
	$image = rwmb_meta( 'custom_field_item', ['size' => 'thumbnail'], 123 );
	return '<img src="' . $image['url'] . '">';
}
add_shortcode('my-shortcode', 'shortcode_function');