This is expected behavior and occurs if the Show on Edit Page checkbox is not checked in the source organization.
The 'Default' checkbox is controlled by:
When deploying case page layout, the checkbox "Show on Edit Page" must be checked for "Default" checkbox to carry over in deployment for Case Assignment Checkbox.
Review the Metadata API Developer Guide for more information about including Layouts in deployments.
Please also refer to below documentation for more details related to deployments using change sets
Special Behavior in Deployments
Components Available in Change Sets
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