it is btn-outline-secondary and not btn-secondary-outline
the class name for outlined buttons is btn-outline-secondary
like in bootstrap samples page
<div class="input-group mb-3">
<div class="input-group-prepend">
<button class="btn btn-outline-secondary" type="button">Button</button>
</div>
<input type="text" class="form-control" placeholder="" aria-label="" aria-describedby="basic-addon1">
</div>
see at https://getbootstrap.com/docs/4.0/components/input-group/#button-addons
and not as used in Modular-admin sample site.
<button type="button" class="btn btn-secondary-outline">Secondary</button>