include-what-you-use not running as part of the build

cmake --version: cmake version 3.15.2

I have a CmakeLists.txt file configured to run include-what-you-use by setting CMAKE_CXX_INCLUDE_WHAT_YOU_USE.

find_program(IWYU include-what-you-use DOC "Path to the include-what-you-use tool")

if (IWYU)
   add_feature_info(include-what-you-use IWYU "Tool to analyze #include usage")
   list(APPEND IWYU -Xiwyu;any;-Xiwyu;iwyu;-Xiwyu;args)
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE ${IWYU})

else ()
   message("Install include-what-you-use to enable the tool")
endif ()

When I perform a build, I do not see any output from include-what-you-use. I have printed CXX_INCLUDE_WHAT_YOU_USE to confirm that the property is set on the target.

If I run the iwyu-tool instead, I do see output.

find_program(IWYU iwyu-tool DOC "Path to the include-what-you-use tool")

if (IWYU) 
   add_feature_info(include-what-you-use IWYU "Tool to analyze #include usage")

   add_custom_command(TARGET 
                      ${PROJECT_NAME}.elf 
                      POST_BUILD 
                      VERBATIM 
                      COMMENT "Running include-what-you-use" 
                      COMMAND "${IWYU}" "-p" ".")
else ()
    message("Install include-what-you-use to enable the tool") 
endif ()

Why is the method of setting the CXX_INCLUDE_WHAT_YOU_USE property not causing the tool to be run?