Help not working if path or environment var is used for QT Installation setting

In the project properties of a Visual Studio project in the category "QT Project Settings" there is a property for the QT installation to be used ("QT Installation"). The help text for this property looks like this:

Qt Installation
Select Qt installation; can be either a path to an installation directory or a version name registered through the Qt Visual Studio Tools (v2.4 or above).

If you actually enter a path here (or use an environment variable like $(MY_QT_DIR)) the integrated context help (pressing F1 on an identifier like "QString") does not work anymore. In the output message window this message appears instead:

No help match was found. You can still try to search online at https://doc.qt.io.

The extension expects an installation name added using the "QT Versions"-dialog for the help to work. But to be able to build a software using QT in a Github-Workflow or using the command line (msbuild) a path or environment variable has to be used because the QT VS Addin is not available there.