Variable interpolation does not work correctly with bootstrap 4, formControl is used before it's defined.

[bwarner]

On line 3073 the definition for inputGroup references formControl before it is defined.
This causes the wrong selector to be used.

Look like this is because the rule on line 3315

.btn-toolbar .input-group {
  width: auto;
}

Causes input-group to be generated before form-control which is defined later in the file.

Here is a screenshot of the css.

[bwarner]

[krainboltgreene]

One way to solve this would be to generate files for each style, and have them inter-import.