qmake Makefile check target creates Windows path

I'm using qmake inside Mingw64/MSYS.

Inside the pro files I need to refer to a relative path (DESTDIR=../bin) so testcase executables are placed correctly.

When running the make check target, the generated Makefile code contains a line with a windows path notation:

{{cd ..\bin && /C/Projects/tests/Testcase/src/target_wrapper.sh ./Testcase.exe }}

Changing to ..\bin will fail, because ..bin is not found:

{{/bin/sh: Zeile 0: cd: ..bin: No such file or directory }}

I can't find the reason why qmake mixes windows and *nix notation.