Android Java Not Updating DynamoDB Table Using Table.updateItem()?

So my table currently has this item with 7 columns.user_id is the primary key.

I do the following to get the current row and update that row with a new List column called "heart_rate_info" added.

    Document newMeasurement = new Document();
    newMeasurement.put("time_ms", model.getTime_ms());
    newMeasurement.put("session_id", model.getSession_id());

    Primitive primitive_user_id = new Primitive(user_id);
    Document currentDoc = userTable.getItem(new Primitive(user_id));
    Document result;
    if(model.getDataType()== MeasurementModel.SENSORS.HEART){
        newMeasurement.put("heart_rate_frequency", model.getRaw_measurement());
        if(currentDoc.get("heart_rate_info")!=null){
            DynamoDBList heart_rate_info = currentDoc.get("heart_rate_info").asDynamoDBList();
            heart_rate_info.add(newMeasurement);
            currentDoc.put("heart_rate_info", heart_rate_info);
            result = userTable.updateItem(currentDoc, primitive_user_id,
                    new UpdateItemOperationConfig().withReturnValues(ReturnValue.UPDATED_NEW));
        }else{
            DynamoDBList heart_rate_info = new DynamoDBList();
            heart_rate_info.add(newMeasurement);
            currentDoc.put("heart_rate_info", heart_rate_info);
            result = userTable.updateItem(currentDoc,
                    new UpdateItemOperationConfig().withReturnValues(ReturnValue.UPDATED_NEW));
        }
    }

When I run the program in debug mode, I can see that it grabs the right row and updates currentDoc with the correct heart_rate_info data. The problem is when I run

result = userTable.updateItem(currentDoc,
                    new UpdateItemOperationConfig().withReturnValues(ReturnValue.UPDATED_NEW));

This returns an empty result. It's not a null value, it doesn't throw an error. Just returns an empty result and the table does not get updated. I'm not completely sure what's going on.

This is currentDoc when I call userTable.get

This is currentDoc right before userTable.updateItem is called.

userTable.putItem 
userTable.getItem

Both work so why won't updateItem work. Any idea?

I'm not an expert on this Java API, but looking at https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/JavaDocumentAPIItemCRUD.html#JavaDocumentAPIItemUpdate it seems that updateItem does not do what you think it does:

If you want to just replace the old item with the new one, use the putItem operation, not updateItem. With updateItem the old item just determines the key of the item, and then you are supposed to use an UpdateExpression to say what gets updated and how. This expression isn't simply an entire new item - see the documentation for its syntax.

Because you didn't give any update expression, the item wasn't updated at all, so the UPDATED_NEW returned only the attributes which the update modified - i.e., nothing.