add_custom_command APPEND does not work in different directories

Usage
1

Is add_custom_command with APPEND supposed to work across different directories? I first suspected it was an issue because I had the main custom command in a sibling directory, but even when defining the main custom command in a root of the APPEND one, it still produces the following error. Moving the APPEND command to the main CMakeLists.txt works correctly.

CMake Error at subdir/CMakeLists.txt:2 (add_custom_command):
  Attempt to APPEND to custom command with output

    E:/src/tmp/cm_append/build/h.h

  which is not already a custom command output.

Setup:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)
PROJECT(cm_app)

add_custom_command(
  OUTPUT ${CMAKE_BINARY_DIR}/h.h
  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/h.h.txt ${CMAKE_CURRENT_BINARY_DIR}/h.h
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/h.h.txt
)
add_custom_target(hh DEPENDS h.h)

add_subdirectory(subdir)

ADD_EXECUTABLE(cm_app main.cpp h.h)
target_include_directories(cm_app PRIVATE ${CMAKE_CURRENT_BINARY_DIR})

subdir/CMakeLists.txt

add_custom_command(
  OUTPUT ${CMAKE_BINARY_DIR}/h.h
  COMMAND echo "second command"
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/b.h.txt
  APPEND
)
2

No. While it isn’t explicitly stated in the docs for APPEND, the output files generated by add_custom_command(OUTPUT) should be considered only visible to things in the same directory scope (loosely speaking, the same CMakeLists.txt file). You can’t APPEND to output files created in a parent or child directory scope.