On TED teams, this temporary solutions comes from LC Verónica Martínez. When searching for users, particularly new ones, sometimes the drop down search box does not display the user you need. By searching by username and not full name and adding parentheses around the username, she was able to find the user and reassign the task.

Example: If searching for Juliana Rincón doesn't give you results, nor searching by her username jules.rincon. write (jules.rincon) and it should work!