Unable to assign users in projects on the Assignee field on the new issue view

Problem Definition

• All of a sudden, some users are unable to assign an issue to other users even though the users have the "Assignable user" and "assign issue" permission. using the Permission helper, you can validate the right permission exist but you're unable to get the desired user on the "Assignee" field

Expected

• A list of users should be visible upon typing a character of a specific user.

Actual Result

• no user list is displayed with the result "No option"
• one or two users are returned on the list. The specific user can't be searched.
  

Workaround

• Check the Screen of the Project for the "Create/Edit/View" and ensure that the "Assignee" Field is added.