When sending a udp packet from an IPv6 socket to an IPv4 mapped address, I would expect the setting for IPV6_TCLASS to be used in the TOS IP header field. It does not appear to be. A similar mapping for TTL should be present, though I did not check for that.

Can be seen with a simple c program and wireshark:

#include <stdio.h>
#include <string.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

int main() {

     int s = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP);
     if (s < 0) {
         perror("socket");
         return -1;
     }

     int value = (40 << 2) & 0xFC;
     int ret = setsockopt(s, IPPROTO_IPV6, IPV6_TCLASS, &value, sizeof(value));
     if (ret < 0) {
         perror("setsockopt");
         return -2;
     }

     struct addrinfo hints;
     memset(&hints, 0, sizeof(hints));
     struct addrinfo *res = NULL;
     ret = getaddrinfo("::ffff:10.10.10.10", "1234", &hints, &res);
     if (ret != 0) {
         perror("getaddrinfo");
         return -3;
     }

     const char *msg = "hello world";
     ssize_t n = sendto(s, msg, strlen(msg), 0, res->ai_addr, res->ai_addrlen);
     if (n < 0) {
         freeaddrinfo(res);
         perror("sendto");
         return -4;
     }

     freeaddrinfo(res);
     return 0;
}