[REQUIRED] Describe your environment

• Operating System version: Windows 10 Home 1909
• Browser version: Chrome 87
• Firebase SDK version: 8.1.2
• Firebase Product: firestore

[REQUIRED] Describe the problem

When getting a document through a transaction (Transaction.get()) the field type reference does not does not return an instance of DocumentReference anymore. Similar to #4125 but with transactions instead of queries.