This change [1] introduced the use of Path.toString() method, which returns OS dependent path string. However MemoryFileSystem works just with "/" path separators, so the compilation cannot find generated source files on Windows, because the paths contain "\" instead of "/".

[1] https://github.com/kiegroup/drools/pull/2117/files#diff-10fdf1615822ed09a09cb551110c1921R118