JIRA User can't be added as a participant in Service Desk customer portal if there's space in the username

Steps to reproduce:

1. Create a JIRA user with a space in the username. (e.g test test)
2. Create a Service Desk ticket in the Customer portal
3. Click on Add people in the portal and add the JIRA user that you've just created. (test test)

Expected Behaviour:

JIRA user is added as a participant

Actual Behaviour:

Throws Error "User cannot be found".

Workaround:

Edit and remove the space in the username.