An issue has been found out regarding sorting forum discussions. Please see details below.

Tested using 3.10.8+ (Build: 20211120) and 3.11.4+ (Build: 20211126)

Replication:
1. Create a course.
2. Enrol three students (using the example users below)

• student01 (mdl_user id: 5)
• student02 (mdl_user id: 6)
• student03 (mdl_user id: 7)
  3. Turn editing on.
  4. Add a Forum activity.
  5. Log in as student01, add a discussion topic and post it. Record the date. Example:
  subject = fromstudent01
  by Student 01 - Monday, 29 November 2021, 12:01 PM
  6. Wait for two minutes, log in as student 02, add a discussion topic and post it. Record the date. Example:
  subject = fromstudent02
  by Student 02 - Monday, 29 November 2021, 12:02 PM
  7. Wait for two minutes, log in as student 03, add a discussion topic and post it. Record the date. Example:
  subject =fromstudent03
  by Student 03 - Monday, 29 November 2021, 12:04 PM
  8. Log in as teacher and visit the forum activity.
  9. Click "Started by". The page will refresh and there will be "Descending" arrow pointing down for the "Started by".
  Result:
  fromstudent03
  fromstudent02
  fromstudent01
  10. Click "Started by" again. The page will refresh and there will be "Ascending" arrow pointing upwards for the "Started by"
  Result:
  fromstudent03
  fromstudent02
  fromstudent01
  11. Go back to the course contents page and click again the forum activity. This is to clear the current sortorder.
  12. Click the "Last post". The page will refresh and there will be "Descending" arrow pointing down for the "Last post".
  Result:
  fromstudent03
  fromstudent02
  fromstudent01
  12 Click the "Last post". The page will refresh and there will be "Ascending" arrow pointing upward for the "Last post".
  Result:
  fromstudent01
  fromstudent02
  fromstudent03

Clicking randomly and different times for Last Post (this sorter mostly works and I reckon seen it not work only about 1 or 2 times while testing) and Started by provides different results. Most times it will not change the sorting. And some random time, it will change sorting correctly.

Additional:
First time a user goes to the Forum discussion page there is no record yet for forum_discussionlistsortorder in the mdl_user_preferences table. When this user visits the forum discussion and clicks the Started by, the sort works, but only for one time. After this, the sorting will not work anymore. A record is added in the mdl_user_preferences table containing the value for forum_discussionlistsortorder. This can not be deleted anymore. It will change once the user attempts to sort the discussion topics.