Why fgets(char* buffer, int size, stdin) don't cause Segmentation Fault all time whenever given input string large enough then size of char buffer[]

Question

consider a typical C program :

   int main()
    {
        char buffer[5];                   
        fgets(buffer, 8, stdin);

        printf("%s", buffer);

        return 0;
    }

I have compiled this code in several machines and various compilers and came to the conclusion that some time compiler gives segmentation fault error and sometime not, but it supposed to give error because our char buffer[5] isn't large enough to hold string of length more than 5. But when I compile this in my machine with input : 12345678 then I'm getting output : 1234567 but I was expecting error : SEGMENTATION FAULT.

I am expecting explanation other than this, Chat-Gpt : "However, the reason why you might not immediately encounter a segmentation fault or error is due to the nature of undefined behavior. Just because you're writing beyond the bounds of the array doesn't necessarily mean the program will crash immediately. The behavior could vary depending on the compiler, the operating system, and other factors." Consider the fact that if we initialize, char buffer[5] = "SAYAN"; and we want to print("%c", buffer[6]); compiler immediately shows error : segmentation fault in this case.