Add Dropdown select of saved data using functions.php

I'm trying a different tack using the Select Advanced field and a callback function.
This is my function, but it's not populating the dropdown yet...

function populate_my_select_field() {
    global $post;
    $options = [];

    // Check if we are in the admin area and a post object exists
    if (is_admin() && !empty($post)) {
        $post_id = $post->ID;

        // Fetch the meta values from the specified group for the current post
        $groups = rwmb_meta('group_j7gfnvh6y4t', '', $post_id);

        foreach ($groups as $group) {
            // Assuming 'height' and 'serial_no' are keys in your group
            $height = $group['height'] ?? '';
            $serial_no = $group['serial_no'] ?? '';

            if ($height && $serial_no) {
                $key = $serial_no; // Using serial_no as the value
                $options[$key] = "Height: $height, Serial No: $serial_no";
            }
        }
    }

    return $options;
}

While I can see my options in the select dropdown, the chosen item is not saved when the post is updated.

I tested with this simplified version:

function populate_my_select_field() {
    return [
        'option1' => 'Option 1',
        'option2' => 'Option 2',
    ];
}

But when I populate using fields from the post, the select box is refreshed each time. How do I get it to commit? One way would be to check through the entries each time in the function and compare to the current value of the dropdown... but is there a more efficient way?

function populate_my_select_field() {
    return [
        'option1' => 'Option 1',
        'option2' => 'Option 2',
    ];
}

And the dynamically populated version:

function populate_my_select_field() {
    $options = [];
    $post_id = null;

    if (is_admin()) {
        $post_id = $_GET['post'] ?? get_the_ID();
    }

    if ($post_id) {
        // Define the groups and their corresponding prefixes
        $group_ids = [
            'group_j7gfnvh6y4t' => 'Anem', // Anemometers
            'vanes' => 'Vane',           // Wind Vanes
            'miscelaneou' => ''        // Miscellaneous
        ];

        foreach ($group_ids as $group_id => $prefix) {
            $groups = rwmb_meta($group_id, '', $post_id);
			// Check if $groups is an array before iterating
            if (is_array($groups)) {
            foreach ($groups as $group) {
                $height = $group['height'] ?? '';
                $serial_no = isset($group['serial_no']) ? trim($group['serial_no']) : ''; // Trim the serial number
                $type = $group_id === 'miscelaneou' ? ($group['type'] ?? 'Unknown') : '';

                if ($height && $serial_no) {
                    // Construct a unique key and label for the option
                    //$key = $group_id . '_' . $serial_no;
                    //$label = ($prefix ?: $type) . " " . $height . "m, Serial No ". $serial_no;
					$key = $serial_no;
					$label = $serial_no;
                    $options[$key] = $label;
                }
            }
		}
        }
    }

    return $options;
}