why "#define BOOST_ASIO_ENABLE_HANDLER_TRACKING 1" doesn't work as expected?

Question

Reference:

https://www.boost.org/doc/libs/1_78_0/doc/html/boost_asio/overview/core/handler_tracking.html https://www.boost.org/doc/libs/1_78_0/doc/html/boost_asio/example/cpp11/handler_tracking/async_tcp_echo_server.cpp

Based on the documentation, when enabled by defining BOOST_ASIO_ENABLE_HANDLER_TRACKING, Boost.Asio writes debugging output to the standard error stream.

I made the following changes to the source code of async_tcp_echo_server.cpp.

 17 #define BOOST_ASIO_ENABLE_HANDLER_TRACKING 1
 18 using boost::asio::ip::tcp;

Above the using boost::asio::ip::tcp;, I added #define BOOST_ASIO_ENABLE_HANDLER_TRACKING 1 and expect this will trigger the debugging track. However, it doesn't work.

$ g++ -std=gnu++2a -Wall -g -ggdb -Werror -I /usr/include/boost -pthread async_tcp_echo_server.cpp -o async_tcp_echo_server
$ ./async_tcp_echo_server 3333
^C

Instead, I removed the added macro definition from the source code and added -DBOOST_ASIO_ENABLE_HANDLER_TRACKING to the command line for the compiler and now it works as expected.

$ g++ -std=gnu++2a -Wall -g -ggdb -Werror -I /usr/include/boost -pthread async_tcp_echo_server.cpp -o async_tcp_echo_server -DBOOST_ASIO_ENABLE_HANDLER_TRACKING
$ ./async_tcp_echo_server 3333
@asio|1645642051.016866|0^1|in 'do_accept' (async_tcp_echo_server.cpp:95)
@asio|1645642051.016866|0*1|[email protected]_accept
@asio|1645642051.017322|.1|non_blocking_accept,ec=system:11

From g++ online manual,

-D name Predefine name as a macro, with definition 1.

Question> Why the macro definition within the source code doesn't work?

Thank you