When manually changing the start date of a task that is not fixed duration, the finish date automatically adjusts after saving retaining the duration. This happens regardless of whether there is an assignment on the task.
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
Component : 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.
See Edit Task Duration in the Gantt Chart of the documentation regarding interactive changes you see when you edit the task attributes on the Gantt view