Aspnetcore: dotnet-watch on project doesn't detect new files

Describe the bug

dotnet-watch on a project as described “Develop ASP.NET Core apps using a file watcher”, and specifically the section Run tests using dotnet watch, does not detect additions of new file.

To Reproduce

Create a directory for the project:

mkdir DotNetWatcherIssueDemo

Change to the project directory:

cd DotNetWatcherIssueDemo

Initialize it with a new test project:

dotnet new nunit

Launch VS Code on the directory containing the test project:

code .

Ask to watch project and run tests on changes:

dotnet watch test

At this stage, you should see the output:

watch : Started
Build started, please wait...
Build completed.

Test run for A:\temp\DotNetWatchIssueDemo\bin\Debug\netcoreapp2.2\DotNetWatchIssueDemo.dll(.NETCoreApp,Version=v2.2)
Microsoft (R) Test Execution Command Line Tool Version 15.9.0
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...

Total tests: 1. Passed: 1. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 0.9364 Seconds
watch : Exited
watch : Waiting for a file to change before restarting dotnet...

Now add a new test to UnitTest1.cs:

diff --git a/UnitTest1.cs b/UnitTest1.cs
index 66ec70e..f6406e7 100644
--- a/UnitTest1.cs
+++ b/UnitTest1.cs
@@ -14,5 +14,11 @@ namespace Tests
         {
             Assert.Pass();
         }
+
+        [Test]
+        public void Test2()
+        {
+            Assert.Pass();
+        }
     }
 }
\ No newline at end of file

dotnet watch will pick up the change, run _two_ tests successfully and continue to watch for further file changes:

watch : Started
Build started, please wait...
Build completed.

Test run for A:\temp\DotNetWatchIssueDemo\bin\Debug\netcoreapp2.2\DotNetWatchIssueDemo.dll(.NETCoreApp,Version=v2.2)
Microsoft (R) Test Execution Command Line Tool Version 15.9.0
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...

Total tests: 2. Passed: 2. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 0.9698 Seconds
watch : Exited
watch : Waiting for a file to change before restarting dotnet...

In a separate shell, cd into the project directory and add a new test file:

dotnet new nunit-test -n UnitTest2 -o .

Expected behavior

At the last step, I expected dotnet watch to pick up the new file change, build and re-run all tests, but it does nothing.

It seems that it builds a static list of files and only watches those for changes. If I touch/force-save the project file (DotNetWatcherIssueDemo.csproj) then dotnet watch detects the change, re-builds the watch list and detects changes to the added file, but again, not for any new file added thereafter.

Additional context

Output of dotnet --info:

.NET Core SDK (reflecting any global.json):
 Version:   2.2.101
 Commit:    236713b0b7

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.2.101\

Host (useful for support):
  Version: 2.2.0
  Commit:  1249f08fed

.NET Core SDKs installed:
  1.1.8 [C:\Program Files\dotnet\sdk]
  1.1.9 [C:\Program Files\dotnet\sdk]
  1.1.10 [C:\Program Files\dotnet\sdk]
  1.1.11 [C:\Program Files\dotnet\sdk]
  2.1.101 [C:\Program Files\dotnet\sdk]
  2.1.103 [C:\Program Files\dotnet\sdk]
  2.1.104 [C:\Program Files\dotnet\sdk]
  2.1.200 [C:\Program Files\dotnet\sdk]
  2.1.201 [C:\Program Files\dotnet\sdk]
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.300 [C:\Program Files\dotnet\sdk]
  2.1.400 [C:\Program Files\dotnet\sdk]
  2.1.402 [C:\Program Files\dotnet\sdk]
  2.1.403 [C:\Program Files\dotnet\sdk]
  2.1.500 [C:\Program Files\dotnet\sdk]
  2.1.502 [C:\Program Files\dotnet\sdk]
  2.2.101 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 1.0.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.0.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.0.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 1.1.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

/cc @natemcmaster