AUTOMOC causes generated files to not be found

cmake_minimum_required(VERSION 3.14)

project(testAutomoc)

find_package(Qt5Core REQUIRED)

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/foo.cpp)

file(GENERATE OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/myConfig.h
    CONTENT
    "
//----
"
)

add_executable(testTarget
    foo.cpp
    ${CMAKE_CURRENT_BINARY_DIR}/myConfig.h
)

# Comment this to make the build work:
set_property(TARGET testTarget PROPERTY AUTOMOC ON)

Cmake output:

$ cmake .. -G Ninja
-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/lib/ccache/cc
-- Check for working C compiler: /usr/lib/ccache/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/lib/ccache/c++
-- Check for working CXX compiler: /usr/lib/ccache/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
CMake Error at CMakeLists.txt:17 (add_executable):
  Cannot find source file:

    /home/stephen/dev/src/playground/cmake/build/myConfig.h

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx


-- Generating done
CMake Generate step failed.  Build files cannot be regenerated correctly.

Note that the file was generated:

$ ls
CMakeCache.txt  CMakeFiles  cmake_install.cmake  foo.cpp  myConfig.h

So, re-running cmake results in success:

$ cmake .
-- Configuring done
-- Generating done
-- Build files have been written to: /home/stephen/dev/src/playground/cmake/build

Every version of CMake I tested (since 3.0.0) is affected. 2.8.12 also fails, but does not generate the file.

This workaround seems to work:

set_property(SOURCE ${CMAKE_CURRENT_BINARY_DIR}/myConfig.h PROPERTY GENERATED TRUE)
set_property(SOURCE ${CMAKE_CURRENT_BINARY_DIR}/myConfig.h PROPERTY SKIP_AUTOMOC TRUE)