If a task has ETC and is mark completed, it will change to a 0 day duration milestone in Microsoft Project (MSP)
Steps to Reproduce:
1. In PPM, create a new project with one team member
2. Create one task, and assign the team member to the task with some ETC
3. Open the project in MSP
4. Change the % Complete to 100 for the task to mark it completed (Note that at this time, the Remaining Work (ETC) then gets reduced to 0)
5. Save the project back to PPM
6. Open the project in MSP again
Expected Results: Task duration doesn't change, and the task is not a milestone
Actual Results: Task duration is changed to 0 days, and the task is then changed to a milestone
This is working as designed. If a task task is marked completed in MSP, any remaining work is deleted (moved to Actual Work). If time is tracked in PPM, the actual work inputted in MSP gets deleted in the save back to PPM, thus when the project is reopened in MSP, if it sees no Remaining Work (ETC) or Actual Work (Actuals) on a task, it will change the duration of the task to 0 (and will make it a milestone) since there is no work to do on it.
This is working as designed as per the information mentioned above.
The best practice is to remove any assignments on a task before marking a task completed if actuals aren't tracked on the task.
Reference section 'Durations Changed Unexpectedly to Zero Days' of Microsoft Project (MSP) Task Integration with CA PPM for more information on the Zero day behavior where this can happen when you also delete the ETC/Remaining work on a task with one assignment.