WordPress nl2br is not converting newline to html line break when saving metabox value

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From the reference/description of sanitize_text_field():

  • Checks for invalid UTF-8,

  • Converts single < characters to entities

  • Strips all tags

  • Removes line breaks, tabs, and extra whitespace

  • Strips octets

So a simple fix is, use sanitize_textarea_field():

The function is like
sanitize_text_field(),
but preserves new lines (\n) and other whitespace, which are
legitimate input in textarea elements.

// In save_custom_metaboxes()
$itinerary = sanitize_textarea_field( $_POST['txtItinerary'] ); // use this one
//$itinerary = sanitize_text_field( $_POST['txtItinerary'] );   // and not this

Additional Notes

Also, you should use esc_textarea() (although the output may not contain HTML tags):

<textarea class="trek_inputs required" name="txtItinerary"...><?php echo // wrapped for clarity
  esc_textarea( $content ); ?></textarea>

And you’d also want to remove line break after each <br />:

$itinerary = sanitize_textarea_field( $_POST['txtItinerary'] );
$itinerary = nl2br( $itinerary );
// Removes line break after each <br />, if any.
$itinerary = preg_replace( "#<br />(\r\n|\n|\r)#", '<br />', $itinerary );