Is accepting 11-char strings but not accepting 19-char strings for constexpr, specified in the C++ specification, or is this just behavior that different compil

Is accepting 11-char strings but not accepting 19-char strings for constexpr, specified in the C++ specification, or is this just behavior that different compiler vendors are implementing differently? If the latter, then it is pretty annoying that non-standard behavior is happening now related to standard libraries implementations and constexpr.