AUTOMOC breaks on configuration-dependent source files

The following project

cmake_minimum_required(VERSION 3.17.0)
project(buggetybug)
set(CMAKE_AUTOMOC YES)
find_package(Qt5 COMPONENTS Core)
add_executable(exe main.cpp
    $<$<CONFIG:Debug>:debug_stuff.cpp>
    $<$<CONFIG:Release>:release_stuff.cpp>)

configured with

cmake -G"Ninja Multi-Config" -DCMAKE_PREFIX_PATH=~/Qt/5.14/lib/cmake '-DCMAKE_CONFIGURATION_TYPES=Debug;Release' 

fails with the very surprising error message

CMake Error in CMakeLists.txt:
  Target "exe" has source files which vary by configuration.  This is not
  supported by the "Ninja Multi-Config" generator.

  Config "Debug":

    /home/jobor/dev/personal/multicfgbug/main.cpp
    /home/jobor/dev/personal/multicfgbug/debug_stuff.cpp

  Config "Release":

    /home/jobor/dev/personal/multicfgbug/main.cpp
    /home/jobor/dev/personal/multicfgbug/release_stuff.cpp

The project builds fine if the line

set(CMAKE_AUTOMOC YES)

is removed / commented out.

This apparently happens, because cmQtAutoGenInitializer::InitScanFiles calls cmGeneratorTarget::GetConfigCommonSourceFiles() to gather the target sources, and this function contains a check that yields above error.

Slightly educated guess: Nothing outside of actual generators should call this function, and cmQtAutoGenInitializer must be - somehow - adjusted to gather the source files in some multi-config-compatible way.

I have attached an example project that exhibits the issue: multicfgbug.tar.gz