CMake's AUTOMOC generates a moc_predefs.h file that it then passes to moc invocations.

That file is generated by calling the CMAKE_CXX_COMPILER_PREDEFINES_COMMAND
which looks something like:

/usr/bin/c++ -dM -E -c <cmake_install_dir>/Modules/CMakeCXXCompilerABI.cpp

This compiler invocation is missing the c++ standard version flag, which can influence the list of predefined defines.

CMake should be fixed to pass the target's specified c++ std version to the predefines command.

Original implementation of predefine generation was done at https://gitlab.kitware.com/cmake/cmake/-/merge_requests/1328/diffs