The author role is very restrictive, users with this role can only see and edit assigned content. In order to be assigned to a workflow task, an author must be assigned to a piece of content either as an author or as a subscriber.

Attribution as author:

Assignment as a subscriber:

Now the author can also be assigned to a workflow task:

NOTE: If an author is already assigned to a task in a workflow template, he or she is automatically listed as a subscriber when new content is created.