Not sure if there is simpler way to solve this, but currently on master, if the wireplumber proxy activation fails while WP_PROXY_FEATURE_BOUND is being activated, then, the created pw_proxy will never be destroyed.
This is because the WP_PROXY_FEATURE_BOUND feature is never set when an error happens, and destruction of the pw_proxy only happens when deactivating a previously set WP_PROXY_FEATURE_BOUND feature.
The leak can be seen with:
valgrind --suppressions=/usr/share/glib-2.0/valgrind/glib.supp --leak-check=full ./build/tests/wp/test-proxy -p /wp/proxy/link_error