This is working as expected. The fixed flag is not taken into account in manual task date changes, the fixed flag is used for Auto Scheduling.
Release : All Supported Releases
Component : CA PPM SAAS PROJECT MANAGEMENT
The Fixed Duration setting is used for Auto Scheduling purposes. Manual changes to the Task Dates are independent of the Fixed setting.
So, changing only the start date will move the task forward without changing the duration regardless of whether the task is Fixed Duration or not.