cmake --install: not working correctly when "CMAKE_INSTALL_LIBDIR" is absolute [NIXPKGS]

[TheRedstoneDEV-DE]

Hello,
I am trying to maintain the drumkv1 nix-package after the previous maintainer has gone and I am facing the following problem:
Everytime I try to build the package, it installs the plugin (so only the library) to "/etc/nix-store/xxxxxxxxx-drumkv1-1.0.0/etc/nix-store/xxxxxxxxx-drumkv1-1.0.0/lib/lv2/.../...", because CMAKE_INSTALL_LIBDIR is set to a absolute path (but all others like CMAKE_INSTALL_BINDIR are set to a relative path... what?), as well as CMAKE_INSTALL_PREFIX (I've figured that out so far; /CMakeLists.txt:51).
When I just set -DCMAKE_INSTALL_LIBDIR=lib it installs completely normal and correctly. I still suspect it is a problem on their side, but they told me: "No both CMAKE_INSTALL_PREFIX and CMAKE_INSTALL_LIBDIR can be absolute paths so it's not an issue at our side! This is an upstream issue.".

My default.nix looks like this at the time (If it matters... I know qt6-full isn't optimal)

  {
    stdenv, lib, pkg-config
  ,  fetchurl
  ,  cmake
  ,  libjack2
  ,  alsa-lib
  ,  libsndfile
  ,  liblo
  ,  lv2
  ,  qt6
  ,  xorg
  }:
  
  stdenv.mkDerivation rec {
    pname = "drumkv1";
    version = "1.0.0";
  
    src = fetchurl {
      url = "mirror://sourceforge/drumkv1/drumkv1-${version}.tar.gz";
      sha256 = "sha256-vi//84boqaVxC/KCg+HF76vB4Opch02LU4RtbVaxaX4=";
    };
  
    buildInputs = [
      libjack2
      alsa-lib
      libsndfile
      liblo
      lv2
      qt6.full
      xorg.libX11
      qt6.qtbase
      qt6.qtwayland
    ];
  
    nativeBuildInputs = [ pkg-config cmake qt6.wrapQtAppsHook ];
  
    cmakeFlags = [
      # disable experimental feature "LV2 port change request"
      "-DCONFIG_LV2_PORT_CHANGE_REQUEST=false"
      # set correct libdir
      "-DCMAKE_INSTALL_LIBDIR=lib"
    ];
  
    meta = with lib; {
      description = "Old-school drum-kit sampler synthesizer with stereo fx";
      mainProgram = "drumkv1_jack";
      homepage = "http://drumkv1.sourceforge.net/";
      license = licenses.gpl2Plus;
      platforms = platforms.linux;
      maintainers = [  ];
    };
  }

Anyways, I don't want to bother you, I know that you are developing this project out of your spare time and I respect your work.
The backstory is, that I depend on drumkv1 working, which it currently isn't because the LV2-Side of the build on nixpkgs is broken (that's definitely not a problem at your side)... I can build it from source and use it, but i figured I could just fix it for everyone... I thought it would be easy, but it clearly isn't that easy...

Thanks in advance!