PHP 7 DOMDocument: utf-8 encoding not working as expected

DOMDocument_encoding_test.php

<?php

// Valid UTF-8 string
$htmlForm = <<<'HTML'
<form action="/" method="post">
    <input type="text" id="title" name="title" required value="Îñţérñåţîöñåļîžåţîöñ" />
    <button type="submit" class="btn btn-primary">Submit</button>
</form>
HTML;

// Check encoding
// Output: UTF-8
var_dump(mb_detect_encoding($htmlForm));

// NOT WORKING !!!!
$dom = new DOMDocument('1.0', 'utf-8');
$dom->loadHTML($htmlForm, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
// Output: // <input value="&Atilde;&#142;&Atilde;&plusmn;&Aring;&pound;&Atilde;&copy;r&Atilde;&plusmn;&Atilde;&yen;&Aring;&pound;&Atilde;&reg;&Atilde;&para;&Atilde;&plusmn;&Atilde;&yen;&Auml;&frac14;&Atilde;&reg;&Aring;&frac34;&Atilde;&yen;&Aring;&pound;&Atilde;&reg;&Atilde;&para;&Atilde;&plusmn;">
var_dump($dom->saveHTML());

// Working option 1
$dom = new DOMDocument('1.0', 'utf-8');
$dom->loadHTML('<?xml encoding="UTF-8">' . $htmlForm, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
// Output: <input value="Îñţérñåţîöñåļîžåţîöñ">
var_dump($dom->saveHTML($dom->getElementsByTagName('form')->item(0)));

// Working option 2
$dom = new DOMDocument('1.0', 'utf-8');
$dom->loadHTML($htmlForm, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
// Output: <input value="Îñţérñåţîöñåļîžåţîöñ">
var_dump(utf8_decode($dom->saveHTML($dom->getElementsByTagName('form')->item(0))));