title: Is mask working differently using inplace?
body: <p>I've just found out about this strange behaviour of mask, could someone explain this to me?</p>
  <p>A)
  [input]</p>
  <pre><code>df = pd.DataFrame(np.arange(10).reshape(-1, 2), columns=['A', 'B'])
  df['C'] ='hi'
  df.mask(df[['A', 'B']]&lt;3, inplace=True)
  </code></pre>
  <p>[output]</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th></th>
  <th>A</th>
  <th>B</th>
  <th>C</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>0</td>
  <td>NaN</td>
  <td>NaN</td>
  <td>hi</td>
  </tr>
  <tr>
  <td>1</td>
  <td>NaN</td>
  <td>3.0</td>
  <td>hi</td>
  </tr>
  <tr>
  <td>2</td>
  <td>4.0</td>
  <td>5.0</td>
  <td>hi</td>
  </tr>
  <tr>
  <td>3</td>
  <td>6.0</td>
  <td>7.0</td>
  <td>hi</td>
  </tr>
  <tr>
  <td>4</td>
  <td>8.0</td>
  <td>9.0</td>
  <td>hi</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>B)
  [input]</p>
  <pre><code>df = pd.DataFrame(np.arange(10).reshape(-1, 2), columns=['A', 'B'])
  df['C'] ='hi'
  df.mask(df[['A', 'B']]&lt;3)
  </code></pre>
  <p>[output]</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th></th>
  <th>A</th>
  <th>B</th>
  <th>C</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>0</td>
  <td>NaN</td>
  <td>NaN</td>
  <td>NaN</td>
  </tr>
  <tr>
  <td>1</td>
  <td>NaN</td>
  <td>3.0</td>
  <td>NaN</td>
  </tr>
  <tr>
  <td>2</td>
  <td>4.0</td>
  <td>5.0</td>
  <td>NaN</td>
  </tr>
  <tr>
  <td>3</td>
  <td>6.0</td>
  <td>7.0</td>
  <td>NaN</td>
  </tr>
  <tr>
  <td>4</td>
  <td>8.0</td>
  <td>9.0</td>
  <td>NaN</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Thank you in advance</p>