Trying to search for mention or assignee in some cases doesn't return correct results

Issue Summary

When someone attempt to search for mention or for assignee by third or following part of display name, returned result differ than what can be expected. For example we cannot search for user Atlassian Jira User by User.

Steps to Reproduce

1. Find project where user can be assigned or mentioned and who has more than two words in display name (like Atlassian Jira User)
2. Try to search in assignee field for this user by at least third part of their display name (User)

Expected Results

User should be included in returned list.

Actual Results

Returned list of users doesn't contain expected user.

Workaround

Currently there is no known workaround for this behaviour. A workaround will be added here when available