cmake add_custom_command POST_BUILD doesn't work as expected

Question

I generate a iOS framework in my project and pass the GCC_GENERATE_DEBUGGING_SYMBOLS to generate dsym for the built framework. The dsym gets generated as expected, but when I try to copy it to another location it doesn't fully work. It copies only the dsym directory structure but not the dwarf file itself or it copies an empty dwarf file.

add_library(${TARGET_OBJC_LIB_NAME} SHARED
    ${DIR_SRCS}
    ${sources}
    ${headers}
    ${objc_sources}
)

add_custom_command(TARGET ${TARGET_OBJC_LIB_NAME}
    POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy_directory $<TARGET_FILE_DIR:${TARGET_OBJC_LIB_NAME}>/.. destinationDirectory
)

I think this is because by the time the TARGET_OBJC_LIB_NAME is built the dsym hasn't fully generated. How do I ask the add_custom_command to wait till the dsym has fully generated?