Makefile generators use wrong relative path for build subdirectories under the source directory but not under the main build directory

cmake version 3.20.0-rc1

When setting FETCHCONTENT_SOURCE_DIR_<ucName> using the generators "NMake Makefile" and "Unix Makefile", the generated CMakeFiles/Makefile2 file mixes up relative path names and absolute path names when referring to the target associated with the set FETCHCONTENT_SOURCE_DIR_<ucName>.

This does not happen when using the "Visual Studio 16 2019" generator and building the resulting solution.

See my main project CMakeLists.txt (rtek_CMakeLists.txt), the dependency CMakeLists.txt (sdslib_CMakeLists.txt) and the generated Makefile2 attached.

Error:

make[1]: *** No rule to make target '../_deps/sdslib-build/CMakeFiles/sdslib.dir/all', needed by 'CMakeFiles/rtek.dir/all'.  Stop.
make: *** [Makefile:91: all] Error 2

Relevant part of my file structure:

repos/
  rtek/
    _deps/
    build/
      CMakeFiles/
        Makefile2
    CMakeLists.txt
  sdslib-cpp/
    CMakeLists.txt

Configure & build steps:

cd repos/rtek
cmake -S. -B./build -G"NMake Makefiles" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DFETCHCONTENT_SOURCE_DIR_SDSLIB="../sdslib-cpp"
cmake --build build

• I tried using an absolute path with FETCHCONTENT_SOURCE_DIR_SDSLIB, but that had no effect.

Relevant parts of Makefile2:

# refers to sdslib.dir\all by relative path name
CMakeFiles\rtek.dir\all: ..\_deps\sdslib-build\CMakeFiles\sdslib.dir\all
# ...
# sdslib.dir\all defined by full path name
C:\Users\stewa\source\repos\rtek\_deps\sdslib-build\CMakeFiles\sdslib.dir\all:
# ...
# sdslib-build/all refers to sdslib.dir/all as absolute path
C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/all: C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/CMakeFiles/sdslib.dir/all

Doing the following find-and-replace in Makefile2 fixes the generated build:

• C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/CMakeFiles/sdslib.dir/all -> ../_deps/sdslib-build/CMakeFiles/sdslib.dir/all
• C:/Users/stewa/source/repos/rtek/_deps/sdslib-build/all -> ../_deps/sdslib-build/all