Build fails intermittently due to missing XML documentation files

When building locally, I see intermittent build errors like:

C:\Code\ironlang\ironpython3\Build\After.targets(42,5): error MSB3030: Could not copy the file "C:\Code\ironlang\ironpython3\bin\Release\IronPython.Wpf.xml" because it was not found. [C:\Code\ironlang\ironpython3\Src\IronPython.Wpf\IronPython.Wpf.csproj]
C:\Code\ironlang\ironpython3\Build\After.targets(42,5): error MSB3030: Could not copy the file "C:\Code\ironlang\ironpython3\bin\Release\netcoreapp3.1\IronPython.Wpf.xml" because it was not found. [C:\Code\ironlang\ironpython3\Src\IronPython.Wpf\IronPython.Wpf.csproj]

Running the build again sometimes may succeed without errors (and sometimes generate more errors). This can also happen with debug builds. I started getting these errors after the upgrade to .NET 5.0, although it seems more a build timing isuse than a functional issue.

Start with Build/After.targets at line 42 and the IronPython.Wpf.csproj referenced by the errors. Reproduce both Release and Debug builds, including a repeated build, and observe when the XML files are missing. Done means the intermittent MSB3030 errors no longer occur.