As I said in my comment, there is a Job Object which is created by ASP.NET core in the out-of-process scenario. The relevant source code portion is here:

https://github.com/aspnet/AspNetCore/blob/master/src/Servers/IIS/AspNetCoreModuleV2/OutOfProcessRequestHandler/serverprocess.cpp#L89

HRESULT
SERVER_PROCESS::SetupJobObject(VOID)
{
    HRESULT                                 hr = S_OK;
    JOBOBJECT_EXTENDED_LIMIT_INFORMATION    jobInfo = { 0 };

    if (m_hJobObject == NULL)
    {
      ....
            jobInfo.BasicLimitInformation.LimitFlags =
                JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE;

            if (!SetInformationJobObject(m_hJobObject,
                JobObjectExtendedLimitInformation,
                &jobInfo,
                sizeof jobInfo))
            {
                hr = HRESULT_FROM_WIN32(GetLastError());
            }
        }
    }

    return hr;
}

As per documentation, JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE:

Causes all processes associated with the job to terminate when the last handle to the job is closed.

If you dig the source further, it does not seem to be optional.

Now, you can actually see this by yourself, if you reproduce your steps. Use Process Explorer, and navigate to your dotnet.exe process, this is what it will display:

• note 1: in fact calc.exe stays alive (at least in my Windows 10 installation because it's now a WinRT app, so it will not end up as child of dotnet.exe), that's why I used notepad.exe
• note 2: iisexpress.exe also creates a job object, but it's configured as breakaway ok, which means it won't kill child processes.
• note 3: if you run from Visual Studio (not my screenshot), you may see an intermediary VSIISExeLauncher.exe process between iisexpress.exe and dotnet.exe. This one also creates a Job Object with 'kill on close' to add to the confusion...