The explicit -UNDEBUG was introduced in the CMake to meson migration (62900cd1) but as far as I can tell it was not present in the CMake tests definition.
First of all, I don't see any reason why we would undef NDEBUG for builds where we explicitly define it.
Secondly, it makes tests less reliable because it enables only some debug messages but not all. For example if emper is build with NDEBUG a test using a Semaphore will output debug messages from the Semaphore but not from the Runtime.