Project Tasks creating milestones (setting duration to 0) in Planning Console and List View whenever the Planned End Date or Planned Duration value is changed. The form view works properly.
This issue was happening due to the "Actual Duration" Planning Console Column not existing in the instance.
1. Project Administration > Settings > Planning Console
2. The Project record for the Default context
3. Check if "Actual Duration" is in the Planning Console Display Columns related list
4. Import or re-create the column if necessary