In my setup i extract path to all test projects using powershell.
i create a string that contains all these paths each separated by a newline " \`n" with a slash '\' to force the filewriting to write the string to read on on newline.
It does not work using " \`r`n".

This data is the loaded into the enviroment using the Environment Injector Plugin and checking the EnvVariable it is a string containing newlines.

The MsTestrunner states that testfiles should be a newline separated list, but only the first entry is separated correct, thus ignoring all other test files.

The fix.
Running the replacement macro on the full testfiles string fixes the problem.
String macroReplacedTestFiles = Util.replaceMacro(testFiles, env);

A fix has been issued to github as a pull request.