UpperCase letter in usernames causes permission failure when user is adding or removing self as reviewer on Pull Request

 Summary

If a username contains an uppercase letter, AND the user has read only permissions to a repository, they are unable to add OR remove themselves as a "Reviewer" for a Pull Request.

Environment

User Directory - Embedded or Remote Directory
Version - Bitbucket Server 5.2.2,  4.14.1,  4.9.1,  4.1.0 

Steps to Reproduce

1. Create User with Capital letter in username field
2. Apply "Bitbucket User" Global Permissions
3. Apply Read Only access Permission to Repository
4. Log in with new user and find active Pull Request in the Web UI
5. Press the "+" symbol for "Add yourself as a reviewer" OR Press the "-" symbol after hovering over your name.

Expected Results

Be added or removed as a Reviewer as only Read Permissions are required. Usernames that are all lowercase can use this feature as expected.

Actual Results

The following pop-up is displayed:

 
The below exception is thrown in the atlassian-bitbucket.log file:

"POST /rest/api/latest/projects/BIT/repos/<Repository Name>/pull-requests/1/participants HTTP/1.0" c.a.s.i.r.e.ServiceExceptionMapper Mapping ServiceException to REST response 401
com.atlassian.bitbucket.AuthorisationException: You do not have permission to update reviewers.

Workaround

As a workaround, the Pull Request can be edited by the creator or those with similar permission to manually add or remove the reviewer on the Pull Request.