According to https://getbootstrap.com/docs/4.1/components/forms/#select-menu the markup for creating a custom select is as follows:

<select class="custom-select">...</select>

When using <b-form-select>, the rendered markup is:

<select class="form-control custom-select">

The inclusion of the .form-control class is incorrect. In fact, it triggers the following Bootstrap CSS which causes the height of the select element to be smaller than it ought to be.

select.form-control {
  &:not([size]):not([multiple]) {
    height: $input-height;
}

The rendered markup of this component should be the same as the Bootstrap prescribed markup.