In the TUNI Moodle version update, the default text editor was changed to the new TinyMCE. It has now been discovered that editing a STACK question with the TinyMCE editor may break the question, because the TinyMCE editor modifies the HTML markup of the question as soon as the question is opened for editing.
Teacher: If you are going to edit or add STACK questions, please change your editor to Atto editor in Moodle:
Your profile picture in top right corner > Preferences > Editor preferences > Text editor: Atto HTML editor