Comment # 2 on bug 1052119 from Wolfgang Bauer

It's a "bug" in the plasma_install_package cmake macro (KF5PlasmMacros.cmake in
plasma-framework), I'd say:
macro(plasma_install_package dir component)
     set(root ${ARGV2})
     set(type ${ARGV3})
     if(NOT root)
        set(root plasmoids)
     endif()
     if(NOT type)
        set(type applet)
     endif()


     kpackage_install_package(${dir} ${component} ${root}
${PLASMA_RELATIVE_DATA_INSTALL_DIR})
     install(FILES ${dir}/metadata.desktop DESTINATION
${KDE_INSTALL_KSERVICES5DIR} RENAME plasma-${type}-${component}.desktop)   <===
endmacro()

This obviously installs the .desktop file to ${KDE_INSTALL_KSERVICES5DIR}...