A forum discussion with unread posts only highlights the first "topic" post and does not highlight unread responses onto it when viewing it "Display replies in threaded form".
Moodle - 3.9.3+ (Build: 20201204)
Forum - 2020061501
1. Create a course and enrol a teacher and student user.
2. Turn editing on.
3. Add a Forum activity.
4. Log in as the teacher user and go to the Forum activity.
5. Add a new discussion topic. Then click reply link.
6. Add a reply to the discussion topic - to create a thread.
7. Log in as the student user and update your Forum preferences to:
7a. Forum tracking = Yes: highlight new posts for me
7b. When sending forum post notifications = Do not mark the post as read
8. Go to the course contents page. You will that beside the forum name, a text is displayed as "2 unread posts".
9. Click the forum activity. Then click the title of the discussion created by the teacher user.
10. You will see that the both the first post (discussion title) and the reply are highlighted. The default view is "Display replies in nested form".
See screenshot forum_nestedform_allhighlighted.png
11. As you have seen the thread, then the highlight will be gone as soon as you leave the page or refresh the page.
12. Change the view to "Display replies in threaded form", so that the next discussion you view, will be displayed in nested form.
13. As teacher add another discussion topic in the same forum and add a "reply onto it" as well.
14. As the student click that discussion topic.
RESULT: You will see that the first post/discussion is displayed in full and it is highighted as yellow.
The reply is beneath it but is not displayed in full and is not highlighted. See screenshot forum_threadedform_only1highlight.png
I can see https://docs.moodle.org/39/en/Using_Forum#Display_options it says:
Display replies in threaded form - Only the post starting the discussion will be displayed in its full form; replies will be reduced to the headlines (including information about its author and date of release) and organised chronologically; moreover, replies will be shifted towards the right so that only replies to the same post were in the same line.
This could be the reason as it only highlights the first post as it is displayed in Full. Perhaps add a text beside to the unread post saying "unread" then highlight it to yellow?