Make sure your demo contains more than one module that contains at least one shared localization string, then check the generated output

What do you mean by module? Like another project?

Exactly, because in Orchard Core, we have a lot of modules and some themes. So, one localization string could be used in many projects that's why the context is handy

The solution now contains a solution with a Web (MVC) and a Razor Class Library (RCL) which is shared.

Food for thought: generate translation template looks like this:

#: PoLocalizationDemo.Lib/Views/Demo/Index.cshtml:6
#. <p>Localized string: @T["Welcome"]</p>
msgctxt "Views.Demo.Index"
msgid "Welcome"
msgstr ""

#: PoLocalizationDemo.Web/Controllers/HomeController.cs:32
#. ViewData["Message"] = T["Welcome from controller"];
msgctxt "PoLocalizationDemo.Web.Controllers.HomeController"
msgid "Welcome from controller"
msgstr ""

#: PoLocalizationDemo.Web/Views/Home/Index.cshtml:8
#. <h1 class="display-4">@T["Welcome"]</h1>
msgctxt "Views.Home.Index"
msgid "Welcome"
msgstr ""

As you can see, the library name is stripped from the views... but this leads me to wonder what would happen if two distinct razor class libraries would have the same view in the same directory and file name? E.g. Views.Home.Index

But, if the msgctxt is prefixed with the project name, the translation is not found by the Orchard localiation infrastructure, which generates the context based on the filename without the project name.