Using memcpy() to write a value different than 0 or 1 into a _Bool is clearly an undefined behavior. Example with clang UBSan.

bool.c:
---
#include <string.h>
#include <stdbool.h>

int main()
{
    char ch = 42;
    _Bool x;
    memcpy(&x, &ch, 1);
    return x == true;
}
---

$ clang -fsanitize=bool bool.c -o bool
$ ./bool
bool.c:9:12: runtime error: load of value 42, which is not a valid value for type '_Bool'