As per quote from the documentation:

Issues added after the sprint starts are indicated with an asterisk.

After do some testing on our JIRA instance, please see the steps:

1. Create a new Scrum board.
2. Create two new issues (Story Points issue type).
3. Create a new Sprint (do not start the sprint).
4. Drag the Issues to the new Sprint (do not add any Story Points values).
5. Then, edit the story estimate points to 10 for SCRUM-1 and SCRUM-2.
6. Now, start the sprint and go navigate to the Sprint Report the values show '-', please see:
   

Finding:

• Its happen to all issue types as well.