title: MongoDB modifiedCount doesn't equal matchedCount
body: <p>I'm doing a simple updateMany in MongoDB.  I'm using Studio3T.  Here is my query:</p>
  <p><code>db.getCollection(&quot;FineMapping&quot;).updateMany( {}, { $rename: { &quot;algorith&quot;: &quot;method&quot; } } )</code></p>
  <p>This is a simple renaming of an attribute over all documents in the collection.  The response in Studio3T is:</p>
  <pre><code>{ 
      &quot;acknowledged&quot; : true, 
      &quot;matchedCount&quot; : 7176215.0, 
      &quot;modifiedCount&quot; : 0.0
  }
  </code></pre>
  <p>So all documents matched, but none are actually modified.  Why aren't the documents actually being modified?</p>