C language makes a difference between Undefined Behavior and Implementation Defined Behavior. In this case it's the former (n1570 section 7.24.2.1).

Any code that invokes UB is not a valid C program, regardless of implemention.

More practically, ignoring the bug that did happen, libc also has multiple implementations of these functions and picks one based on HW it is running on. So even a statically linked glibc could behave differently on different HW. Always read the docs, this is well defined in the standard.