Usernames with capital letters disable 'Basic' search

Symptom

"Basic" search button not available in issue navigator where JQL query contains a username with capital letters.

Steps to reproduce

1. Create an user with capital letter(s) in name (e.g. Testuser)
2. Browse to the issue navigator and search for this user either in the "reporter" or "assignee" field
3. You will be able to search either in Basic or Advanced mode
4. In the issue navigator using Advanced mode, search for the user either in the "reporter" or "assignee" field
5. The "Basic" search will be disabled
6. If you hove on the "Basic" link, the message "This query is too complex to display in Simple mode" will be shown

Expected behavior

Be able to search in Basic and Advanced mode

Current behavior

Basic search is disabled.

Workaround

In the JQL make the capital letters lowercase in the username (e.g. testuser)