We would like to report an issue where fullDocument field in ChangeEvent is not being populated every time and is NULL at times, for 'Update' operation type.
Can you please let us know if it is an issue in MongoDB or it is expected to be NULL in certain scenarios (apart from when document is deleted or collection is dropped or renamed)?
 
Environment Details:
mongos: 3 servers (with 3 config servers)
mongod: 3 sharded replica sets, with each having one primary, two secondaries, and one arbiter
Mongo version: 4.0.5
Client Java Driver version: 3.10.1
 
Write concern: w:1 (default)
Read preference: Primary
Read concern: Not configured (default)
 
Operations:
Insert a document with TxId:1234
Update-1 on inserted document
Update-2 on inserted document
Update-3 on inserted document
 
Issue: 
Client is rightly receiving the Insert events and is able to find the document in fullDocument field,
However it is not finding document in every update change event in fullDocument field, and at times it is NULL
 
Connector watches change streams for Insert and Update events
mongoCollection = mongoClient.getDatabase(mongodb_database).getCollection(mongodb_collection) .withReadPreference(ReadPreference.primary());
cursor = mongoCollection.watch(pipeline).fullDocument(FullDocument.UPDATE_LOOKUP).iterator();
 
In poll method:
             ChangeStreamDocument<Document> next = cursor.tryNext();
             if (null != next) {
Document fullDocument = next.getFullDocument();
  (This field is NULL)
 
Questions:
1. As per documentation we found, fullDocument is expected to be null when a document is deleted or a collection is dropped or renamed, but in our cases, neither of this is done.
We would like to understand why this field isn't being populated.
 
2. As write and reads are from primary, and change event being generated inherently after 'majority' commit, is there a possibility that reads might be going to secondary?
 
3. We have a similar infrastructure with Mongo version 3.6, and are not observing these issues.  Can you share if any changes with migration to 4.0.5 are causing this?