IIUC, your discovery relates to

> If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.

This wording has been challenged by at least these papers:

http://open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3801.pdf (make it ill-formed instead of undefined)

http://www.open-std.org/JTC1/SC22/wg21/docs/papers/2014/n3882.pdf (make execution of the directive "conditionally supported")

but as of C++20, it's still UB.

–Arthur