The Microsoft Project (MSP) connector / Interface allows Tasks with Actuals, Estimates and Assignments to be saved back to Clarity as milestones. Summary tasks are not allowed to be saved back with assignments and instead an error is generated: Summary task assignments are not supported, should milestones have the same restriction/error?
This is working as designed that milestones with assignments can be saved back to Clarity with no error. Summary tasks behavior is configured differently with the MSP Interface than that of milestones. While you are not able to make a task a milestone in Clarity if it has an assignment, the behavior is different in working with the MSP Interface, and this is documented at: Task, Summary Task, and Milestone Configurations
Release : All Supported
Component : CA PPM SAAS MICROSOFT PROJECT (MSP)
See Task, Summary Task, and Milestone Configurations for detailed information on milestone behavior in integrating with MSP, but below is a diagram with high level details from the documentation. As per the below diagram: