As reported in the support forum, given post content that contains this markup:

<!-- wp:code -->
<pre class="wp-block-code"><code>if (FELTÉTEL)
{
  IGAZ ÁG
}
else
{
  HAMIS ÁG
}</code></pre>
<!-- /wp:code -->

The result on the frontend is:

Note how “ÁG” shows up as “ÁGG”, and how “FELTÉTEL” ends up as “FELTÉTELL”. The last character is duplicated.