In mod/quiz/renderer.php starting at around line 1086, there is this code:

if ($viewobj->feedbackcolumn) {

 $resultinfo .= $this->heading(get_string('overallfeedback', 'quiz'), 3, 'main');

 $resultinfo .= html_writer::tag('p',

 quiz_feedback_for_grade($viewobj->mygrade, $quiz, $context), array('class' => 'quizgradefeedback'))."\n";

}

which generates the feedback for the actual quiz. In my case, the feedback entered by the teacher is something like <p>foo bar barfu</p>, whilst the text itself doesn't matter, it's the surrounding <p>-tags, generated by TinyMCE. The code above with the entered feedback populates to the following HTML:

<div id="feedback" class="box generalbox">

<h2 class="main">Bester Versuch: 5,00 / 10,00.</h2>

<h3 class="main">Gesamt-Feedback</h3>

<p class="quizgradefeedback"></p>

<p>Sie haben das Modul abgeschlossen.</p>

<p></p>

</div>

As one can see, the feedback isn't in the right spot, which should be the paragraph with the class I suppose. I didn't dig deeper in the html_writer class, but I'm guessing it's not nesting the paragraphs, hence the "wrong" HTML code above. I suggest changing the shown php code from html_writer::tag('p' to html_writer::tag('div' as it behaves as expected.