QtCreator: cmake source_group not working properly

#              "" ->empty named group, i.e. toplevel - directly under target
  source_group("" FILES
    ${CMAKE_BINARY_DIR}/src/a/a1.cpp
    ${CMAKE_BINARY_DIR}/src/a/a2.cpp
    )
  source_group("" FILES
    ${CMAKE_BINARY_DIR}/inc/a/a1.h
    ${CMAKE_BINARY_DIR}/inc/a/a2.h
    )

cmake_minimum_required(VERSION 3.18)

project(go LANGUAGES CXX)

file(WRITE ${CMAKE_BINARY_DIR}/src/a/a1.cpp "")
file(WRITE ${CMAKE_BINARY_DIR}/src/a/a2.cpp "")

file(WRITE ${CMAKE_BINARY_DIR}/inc/a/a1.h "")
file(WRITE ${CMAKE_BINARY_DIR}/inc/a/a2.h "")

add_library(a
  ${CMAKE_BINARY_DIR}/src/a/a1.cpp
  ${CMAKE_BINARY_DIR}/src/a/a2.cpp
  ${CMAKE_BINARY_DIR}/inc/a/a1.h
  ${CMAKE_BINARY_DIR}/inc/a/a2.h
  )

set(SG_VARIANT "1a" CACHE STRING "variant")
set(SG_VARIANT_CHOICES "1a" "1b" "2a" "2b")
set_property(CACHE SG_VARIANT  PROPERTY STRINGS ${SG_VARIANT_CHOICES})
if (NOT SG_VARIANT IN_LIST SG_VARIANT_CHOICES)
  message(FATAL_ERROR "SG_VARIANT must be one of ${SG_VARIANT_CHOICES}")
endif ()
message(STATUS "SG_VARIANT " ${SG_VARIANT})

if ("${SG_VARIANT}" STREQUAL "1a")
  ## variant 1a
  source_group("" FILES
    ${CMAKE_BINARY_DIR}/src/a/a1.cpp
    ${CMAKE_BINARY_DIR}/src/a/a2.cpp
    )
  source_group("" FILES
    ${CMAKE_BINARY_DIR}/inc/a/a1.h
    ${CMAKE_BINARY_DIR}/inc/a/a2.h
    )
elseif ("${SG_VARIANT}" STREQUAL "1b")
  ## variant 1b
  source_group(TREE ${CMAKE_BINARY_DIR}/src/a FILES
    ${CMAKE_BINARY_DIR}/src/a/a1.cpp
    ${CMAKE_BINARY_DIR}/src/a/a2.cpp
    )
  source_group(TREE ${CMAKE_BINARY_DIR}/inc/a FILES
    ${CMAKE_BINARY_DIR}/inc/a/a1.h
    ${CMAKE_BINARY_DIR}/inc/a/a2.h
    )
elseif ("${SG_VARIANT}" STREQUAL "2a")
  ## variant 2a
  source_group("here\\sources" FILES
    ${CMAKE_BINARY_DIR}/src/a/a1.cpp
    ${CMAKE_BINARY_DIR}/src/a/a2.cpp
    )
  source_group("here\\includes" FILES
    ${CMAKE_BINARY_DIR}/inc/a/a1.h
    ${CMAKE_BINARY_DIR}/inc/a/a2.h
    )
elseif ("${SG_VARIANT}" STREQUAL "2b")
  ## variant 2b
  source_group(TREE ${CMAKE_BINARY_DIR}/src/a PREFIX "here\\sources" FILES
    ${CMAKE_BINARY_DIR}/src/a/a1.cpp
    ${CMAKE_BINARY_DIR}/src/a/a2.cpp
    )
  source_group(TREE ${CMAKE_BINARY_DIR}/inc/a PREFIX "here\\includes" FILES
    ${CMAKE_BINARY_DIR}/inc/a/a1.h
    ${CMAKE_BINARY_DIR}/inc/a/a2.h
    )
endif()

cmake -G "Visual Studio 17 2022" -D SG_VARIANT=1a ../bugreport
cmake --open .