SUMMARY
I am trying to migrate a Kate plugin from Qt5.15.* to 6.1. The CMakeLists.txt is calling 
kcoreaddons_add_plugin(my_plugin_name 
    INSTALL_NAMESPACE "ktexteditor"), 
which with Qt6 builds the sources but fails upon linking because it is trying to link not in the build folder but in 
/INSTALL_NAMESPACE, i.e. in 
/ktexteditor

(For Qt6 the install namespace should be kf6/ktexteditor).

STEPS TO REPRODUCE
In $project_dir I run:
rm -rf build/
cmake -B build/ -D CMAKE_BUILD_TYPE=Release -D QT_MAJOR_VERSION=6 (or 5)
cmake --build build/

This is working fine with Qt5 and will create ./build/plugin_name.so in the build folder. A subsequent 
sudo cmake --install build/
would then copy the plugin .so to 
/usr/lib/qt/plugins/ktexteditor/

With Qt6 I am getting a linker error:
cmake --build build/
[......]
[ 85%] Linking CXX shared module /kf6/ktexteditor/my_plugin.so
/usr/bin/ld: cannot open output file /kf6/ktexteditor/my_plugin.so: No such file or directory
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/my_plugin.dir/build.make:153: /kf6/ktexteditor/my_plugin.so] Error 1
make[1]: *** [CMakeFiles/Makefile2:653: CMakeFiles/my_plugin.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

OBSERVED RESULT
cmake --build build/ 
attempts to link the plugin .so in /kf6/ktexteditor/

EXPECTED RESULT
cmake --build build/ 
should link the plugin .so in ./build/

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro 
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFORMATION