Dynamically Populate Both Labels and Values for ACF Select Field

For achieving this we need to build the $options array in a way that each key-value pair represents the label and the value for the select field.

Here is the updated code, which you need to use instead of your code.

function populate_itnday_select_field( $field ) {
    // Here we have Initialize the options array.
    $options = array();
    
    // Here we are checking if the flexible content field has rows of data.
    if ( have_rows( 'rec_data' ) ) {
        // Loop through the rows.
        while ( have_rows('rec_data') ) {
            the_row();
            
            // Here we are checking if the current layout is the one we want to extract data from.
            if ( get_row_layout() == 'pageday_title' ) {
            
                // Here we are getting the label and value. 
                // Here we are assuming 'daytitle_headline' is the label and you need a unique value for the select options.
                $dayTtlNav = get_sub_field('daytitle_headline');
                
                // Here we are using the label as both key and value.
                $options[ $dayTtlNav ] = $dayTtlNav;
                
                // Here we can modify the key, If we need a different value.
                // $options[$unique_value] = $dayTtlNav;
            }
        }
    }

    // Here we have set the select field choices.
    $field['choices'] = $options;
    
    return $field;
}

add_filter( 'acf/load_field/name=int_event_day', 'populate_itnday_select_field' );