There is a flaw in the underlying msbuild tasks that are used by Visual Studio when the XML documentation file is generated in the same directory as the one specified as the output path, for intermediary builds.

To fix it, you need to specify a different directory, for example, like this:

and change your post-build copy command accordingly, like this:

copy /y "C:\Projects\MyProject\MyProject\docbin\Debug\MyProject.XML" "C:\TEMP\"