Character array size check for stack smashing

Question

In the code below, I'm attempting to concatenate 2 strings: str1 and str2 using strcat from string.h

Since the length of destination string, str1 is 6 bytes on the stack, I expect any store of a length more than 6 shall trigger the stack smashing detected warning message. However, I see that the message is triggered only in case where Im attempting to write 8 bytes (12345,67,NUL) which is what strcat will do when called as strcat(str1, str2)

Surprisingly, a write of 7 bytes (12345,6,NUL) does NOT trigger the stack smashing detected warning, this seems wrong. I realize that strcat specifies undefined behavior when destination size is insufficient, but why does it affect the stash smashing check?

#include <stdio.h>
#include <string.h>

int main() {
    char str1[6] = "12345";
    char* str2 = "6"; // works fine, but why?
    //char* str2 = "67"; // gives "*** stack smashing detected ***: terminated"
    strcat(str1, str2);
    return 0;
}

gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0