I’ve found the problem:
At the file
class-instant-articles-post.php
line 638
I’ve added utf8_decode to $title variable:
$document->loadHTML( '<?xml encoding="' . $blog_charset . '" ?><h1>' . utf8_decode($title) . '</h1>' );
And now it’s working corrrectly.
I don’t know if this is the best approach, but it worked for me.
(version 3.1.3)