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.