<EnableDefaultCompileItems>false</EnableDefaultCompileItems> partially working

**Visual Studio Version**:
VisualStudio 2019 v16.9.0

**Summary**:
The `false` MSBuild property is not completely working. Specifically, using it, in combination with platform specific `` Items, results in filtering platform specific files in some cases but not others.

Sample Project: https://github.com/baskren/VisualStudio.CrossPlatformProjects1.git

In the above solution, there are 8 platform specific Library projects:

For common build properties and items, there is a `Directory.Build.props` file in the same folder as the platform specific projects for the library. It is as follows:
``` xml

true
false

CrossPlatformLibrary1
CrossPlatformLibrary1

Uno0001

bin\$(MSBuildProjectName)
obj\$(MSBuildProjectName)

false
false
$(DefaultItemExcludes);obj/**;bin/**

portable

true

```

Notice the `false` on line 17 (above).

Each platform specific project for the library looks something like this:
``` xml

monoandroid10.0

true
5

```

The below screen shot illustrates the issue:

Notice that some CrossPlatformLibrary1 platform projects (Skia.Gtk, Skia.Tizen, Skia.Wpf, and Wasm, in this example) are correctly filtering where as the other projects (Android, iOS, MacOS, and UWP) are not.

**Steps to Reproduce**:

1. Open the solution found at : https://github.com/baskren/VisualStudio.CrossPlatformProjects1.git

2. In Solution Explorer, examine the platform projects for each platform project in the Library folder

**Expected Behavior**:

Mis-matched platform specific files are filtered out of each platform project (ex: Class1.ios.cs does not appear in the CrossPlatformLibrary1.Android project listing in Solution Explorer

**Actual Behavior**:

Mis-matched platform specific files are **NOT** filtered out of each platform project (ex: Class1.ios.cs, as well as others, appears in the CrossPlatformLibrary1.Android project listing in Solution Explorer

**User Impact**:

Low to moderate. Fortunately, the build process IS WORKING. This is a VisualStudio 2019 v16.9.0 issue (it was not happening in last release of version 16.8).