Assigning labor resource to task in Manage Project Plan page

Summary:

When we are assigning labor resource to a task in Manage Project Plan page in Task details, the task is automatically getting converted to Milestone with zero duration. Is this the standard behavior ? How can we assign labor resource (person) to project task without task attribute getting changed to milestone. We need the person to be assigned to tasks to update the respective task progress. Let me know how can we achieve this.

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