Single toggle button used to work with checkbox/radio buttons on 4.0.0-beta.2 like this:

<div data-toggle="buttons">
  <label class="btn btn-outline-primary">
    <input type="checkbox" autocomplete="off"> I don't know
  </label>
</div>

Output:

But it doesn't work anymore on 4.0.0-beta.3:

I searched your change log and found #23728 but It doesn't make sense to add btn-group-toggle to a single toggle button to make this work.