When using the JSON-based version of the Redmine REST API for CRUD operations of issues, the JSON key for the assignee of the issue is not unique:
for creation, one needs to use "assigned_to_id"; when retrieving issues, though, the assignee's id is transmitted using the key "assigned_to".

In java-based clients (for instance the Redmine Java API), this makes usage of Gson for parsing and creating JSON code that is accepted by Redmine pretty hard, as Gson uses a (single) annotation on the members for specifying that kind of serialization information.