When the status is set to completed for a task with a resource is assigned and the project is opened with Microsoft Project, the MSP 'duration' field is changed to zero.
The task has an assignment with ETC, but after setting the task to 100% complete (and Status = Complete), the ETC is zeroed out - as the task is complete, there could not be remaining work to complete.
If the task does not have any Actuals, as the duration would be the Work, such as ETC (Remaining Work) + Actuals (Actual Work), and both are set to zero, the duration will be zero as well.
Set the Actuals for the task as appropriate if there has been any work completed for the task.