“no memory issues in the tested state space”. That’s the only thing Valgrind can say. But it says nothing how a run with different input would behave, it just might segfault/leak/use after free/UB.

That is always the case in any platform. Just because something works on a Mac it will not necessarily work on a PC or vice-versa. If a language has multiple compilers, you also need to test in different compilers to make sure your code works there too. You're trying to make this as a C-only issue, when it is a general issue, maybe with different names.