Issue search by using displayName value in JQL no longer works

Summary

Use a user's display name as part of JQL no longer works and says that the value does not exist for the field. For example

assignee = "Ian Gil Ragudo"

Steps to Reproduce

1. Via UI go to issue search
2. Advance view
3. Use a field that requires a user like assignee or reporter and key in the display name/full name of the user

Expected Results

Display name will be recognized as a valid value for the field and search results will be returned.

Actual Results

• Via Search REST API

  {
      "startAt": 0,
      "maxResults": 50,
      "total": 0,
      "issues": [],
      "warningMessages": [
          "The value 'Ian Gil Ragudo' does not exist for the field 'assignee'."
      ]
  }
  

  with a status of 200

• Via issue search in UI

  "The value 'Ian Gil Ragudo' does not exist for the field 'assignee'."
  

Notes

• Does not happen to all cloud instances
• For affected instances, it can be consistently replicated
• First noticed on March 28, 2019