To protect users' privacy, sometimes you may not be able to see another user's name, instead seeing an "Unknown" user.
Typically this happens between member and guest users. All workspace members know each other and will always see each others' names on a task.
Here are some scenarios in which you might see an "Unknown" user:
You're viewing a task that's added to list A and list B. You have access to list A, but not list B, and a guest user only has access to list B. Because you don't share a list in common, you will both see each other as "Unknown" user.
You're viewing a task that you only have access to through task-level granted permissions (e.g. you were added directly to the task permissions). You do not have access to any of the task's lists and will see guests that have access to that task via list permissions as "Unknown".
You're viewing a task that you have access to through list A and that a guest user has access to directly via the task's share menu (not from any lists).
If you don't share explicit access to the task or list, instead of seeing these users' names or avatars, you'll see <Unknown> user in places like: Assignees, Subscribers, Read receipts & typing indicators, chat and activity history, and the Share menu, etc.
How it works
Workspace admins know all of their workspace's users, including guests, and will see all users' identities everywhere across the app.
All workspace members will see fellow members' names and avatars. Members only know about guests if they've been explicitly added* to the same list or same specific task.
Guests only know about other users (both members and other guests) on lists or tasks that they've both been explicitly added to.
Logged out users viewing a list via "Public access" URL will not see the identities of any users on that list.
*Explicitly added means that they've been invited to the list or task by name, not just through "All members" access or "Web" access.