Why is modifiedCount missing in Mongo bulkWrite() result?

I'm on mongo v4.2. See the below mongo shell script which is an exact copy of the example from mongo bulkWrite related documentation

db.pizzas.insertMany( [
   { _id: 0, type: "pepperoni", size: "small", price: 4 },
   { _id: 1, type: "cheese", size: "medium", price: 7 },
   { _id: 2, type: "vegan", size: "large", price: 8 }
] )

try {
   db.pizzas.bulkWrite( [
      { insertOne: { document: { _id: 3, type: "beef", size: "medium", price: 6 } } },
      { insertOne: { document: { _id: 4, type: "sausage", size: "large", price: 10 } } },
      { updateOne: {
         filter: { type: "cheese" },
         update: { $set: { price: 8 } }
      } },
      { deleteOne: { filter: { type: "pepperoni"} } },
      { replaceOne: {
         filter: { type: "vegan" },
         replacement: { type: "tofu", size: "small", price: 4 }
      } }
   ] )
} catch( error ) {
   print( error )
}

The actual bulkWrite result returned is as follows

{ 
    "acknowledged" : true, 
    "deletedCount" : 1.0, 
    "insertedCount" : 2.0, 
    "matchedCount" : 2.0, 
    "upsertedCount" : 0.0, 
    "insertedIds" : {
        "0" : 3.0, 
        "1" : 4.0
    }, 
    "upsertedIds" : {

    }
}

You will note that the modifiedCount field mentioned in the documentation is missing. Why is it not included? Is matchedCount an alternative?

I also tried another much simpler script to reproduce this

db.someCollection.insertOne({ "_id": 1, "field": "some-value" })    
var result = db.someCollection.bulkWrite([{
    updateOne: {
         filter: { "_id": 1 },
         update: { "$set": { "field": "another-value" } },
         upsert: false   
    }
}])
printjson(result)

I see the same behavior. No modifiedCount.

{ 
    "acknowledged" : true, 
    "deletedCount" : 0.0, 
    "insertedCount" : 0.0, 
    "matchedCount" : 1.0, 
    "upsertedCount" : 0.0, 
    "insertedIds" : {

    }, 
    "upsertedIds" : {

    }
}