title: Mongo driver updateMany not updating all documents
body: <p>Using Mongo Java Driver version 3.2.1 against MongoDB 3.0.12.</p>
  
  <p>Calling <code>MongoCollection.updateMany(Bson filter, Bson update)</code> returns a result showing all expected documents were modified, however only a portion of the documents were actually updated. </p>
  
  <p>I've tried with multiple write concerns: JOURNALED, ACKNOWLEDGED, etc</p>
  
  <p>Any ideas?</p>
  
  <p>Here is the profile result:</p>
  
  <pre><code>{ "op" : "update", "ns" : "dev.timeSheet", "query" : { "lineItems.task" : ObjectId("53233e85e4b07f573f1d4466") }, "updateobj" : { "$set" : { "lineItems.$.task" : ObjectId("53233e85e4b07f573f1d446d") } }, "nscanned" : 0, "nscannedObjects" : 6733, "nMatched" : 248, "nModified" : 248, "fastmod" : true, "keyUpdates" : 0, "writeConflicts" : 0, "numYield" : 52, "locks" : { "Global" : { "acquireCount" : { "r" : NumberLong(53), "w" : NumberLong(53) } }, "MMAPV1Journal" : { "acquireCount" : { "w" : NumberLong(301) } }, "Database" : { "acquireCount" : { "w" : NumberLong(53) } }, "Collection" : { "acquireCount" : { "W" : NumberLong(53) } } }, "millis" : 50, "execStats" : {  }, "ts" : ISODate("2016-08-25T18:17:16.025Z"), "client" : "127.0.0.1", "allUsers" : [ ], "user" : "" }
  </code></pre>
  
  <p>Update: Also occurs in MongoDB 3.2.9
  Direct access calls:</p>
  
  <pre><code>db.timeSheet.find({'lineItems.task': ObjectId("53233e85e4b07f573f1d4466")}).count()
  126
  db.timeSheet.updateMany({'lineItems.task': ObjectId("53233e85e4b07f573f1d4466")}, {'$set': {'lineItems.$.task': ObjectId("53233e85e4b07f573f1d446d")}})
  { "acknowledged" : true, "matchedCount" : 126, "modifiedCount" : 126 }
  db.timeSheet.find({'lineItems.task': ObjectId("53233e85e4b07f573f1d4466")}).count()
  90
  </code></pre>