Steps to reproduce:

1. Open Unity Editor
2. Open 'Preferences/External Tools' window
3. Under 'Xcode Default Settings' enable 'Automatically Sign' checkbox and enter a valid 'Signing Team ID'
4. Close the editor, re-open and create a new project
5. Switch to iOS
6. Open 'Player Settings/Other' window
7. Notice the 'Team ID' field is set correctly
8. Build the project
9. Open in Xcode
10. Team ID is not set in Xcode project

Changing something in the 'Team ID' field in Unity player settings (like adding and deleting a random character), will force the field to be re-serialised and then the Team ID will be set correctly.

Reproduced with: 2018.1.0a6
Works correctly on 2017.3.x and earlier