I could reproduce this with Visual Studio 2017 (15.9.15) as well. => Not related to the VS version.

It looks like it is a problem with the new "SDK" style *.csproj file format.

See attachment "OldProjectFormat.zip" for an example using the old file format that works as expected.

Only change I had to make in the post-build event is that the new "SDK" style has the project directory as start for relative paths while the old style starts directly from the bin\debug folder. Perhaps the issue is related?