This occurred because you likely have the System Setting set to Work Policy = Fixed Duration. When adding new work items, this is what is used. Note that if you have child tasks beneath the Milestones are they Fixed Work or Duration? Note that this is a setting that you configure per object. If you specify the Project as Fixed Work, all other sub-work items will not automatically change and new sub work items will still use the default system setting for Work Policy for new Work Items.
This is System Setting:
|7.2||Work policy behavior (System Default = Fixed Duration)|