Activity records cannot be located in a full Sandbox after a refresh but these records are available in the production org.
Salesforce allows for archiving activity records but does not copy the archived records over when performing a refresh.
This is working as designed.
Salesforce Help Documentation: Considerations for Archiving Activities
Send us your feedback: We're always looking for advice to help improve our Knowledge Base! Please let us know if this article was helpful or provide feedback on how we can improve your experience here.