It is not possible to access any of the user property (default ones as the firstname, lastname, email...) of custom one (like manager) in a workflow context. We would need it to assign some task according to task performer properties. The only way to do it currently is by doing a walkaround and store the information within a document property (https://doc.nuxeo.com/nxdoc/how-to-assign-a-task-to-the-user-manager/).
This ticket is related to https://answers.nuxeo.com/general/q/8c94994a7080491a9229c6cd538a99ec/How-to-Assign-a-Task-to-the-User-Manager where all different tries are listed with the corresponding stack trace.
Ticket created following a discussion with Alain Escaffre.