Bootstrap 4 invalid feedback with input group not displaying

Boostrap 4 is very buggy. My suggestion is to replace:

 <div class="invalid-feedback">
 Text here
 </div>

With:

<div class="text-danger">
Text here
</div>

And the second one looks virtually the same and will not fail.

For a better look, try:

<div class="text-danger">
<small>Text here</small>
</div>