Question:

What happens when promoting an Inbox Item to multiple Cases in Safety Vault?

Answer:

The first case record that is created is locked as the other case records are created asynchronously.  After the other case records are created, the first case unlocks. 

Please note if there are JavaSDK solutions or configurations that involve a Case object field value change for inbox items promoted to multiple cases, the field value change can result in an error for the first case if the field value change is attempted while the case is locked. However, the field value change is successful for the other cases.  

Related Documentation:

Vault Safety Help Documentation: About Multiple Case Promotion