Parameter accepted ... ... or not

Subject: Bootstrap 4.

Problem: 

The parameter "form-control" is, although compliant, declared in email fields as "not allowed" and also not displayed in the browser. Why?
As you can see, this parameter is accepted in the above div. According to documentation, this should actually be the case with "type = email". I do not understand the behavior of PHPStorm here. Version is 2018.2.3. Bootstrap is version 4 via CDN.