Teachers can't see essay questions properly when grading (if there is HTML in the question text, for example, image or table).
There is an issue in the mod/lesson/essay.php(line 404,405): function format_string is used to prepare texts. This function removes all HTML code from the text.

Since students are able to use WYSIWYG editor for answering the essay questions, format_string function should be changed to format_text function for both question and answer (lines 404 and 405).