UserName not present on external users

[JakeStanger]

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

SharePoint REST API

Developer environment

Linux

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS)
• mobile (Android)
• not applicable
• other (enter in the "Additional environment details" area below)

Additional environment details

• FireFox: 88.0.1 (64-bit)

Describe the bug / error

Several external users are being returned with a null UserName field when expanding a Person list column for list items.

Due to #4478 when using multi-person fields this causes requests selecting the users to fail.

This seems to be affecting one tenant exclusively at the moment, who have ~4500 total users.

I have also noticed that for the OOTB list people picker, the user does not show up.

Steps to reproduce

1. Create list with Person field
2. Create item with external user in Person field (using API)
3. Run the following:

await fetch("/_api/web/lists/getByTitle('YOUR TEST LIST')/items/getById(1)?$expand=Person&$select=Person/UserName", {
  headers: {
    accept: "application/json"
  }
}).then(r => r.json())

4. Observe that the UserName field is null.

Expected behavior

The UserName field contains the user's login email address, much like it does for regular users.

Thank you for reporting this issue. We will be triaging your incoming issue as soon as possible.

[github-actions]

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