just as a followup to this, I have read that IPv4 mapped addresses in traceroutes are caused by MPLS routers in the path that do not have IPv6 native stacks.
AT&T would share the Cogent "incompetence", likely for the same reasons;
traceroute6
www.ietf.orgtraceroute to
www.ietf.org (2001:1890:123a::1:1e), 30 hops max, 80 byte packets
1 cconnv6.cconn.info (2605:2a00:ffff:fffd::1) 3.812 ms 6.499 ms 7.446 ms
2 v6.hautevitesse.net (2605:2a00:ffff:ffff::1) 4.545 ms 4.667 ms 4.783 ms
3 2001:550:2:16::9 (2001:550:2:16::9) 4.072 ms 4.165 ms 4.285 ms
4 2001:1890:1fff:105:192:205:36:77 (2001:1890:1fff:105:192:205:36:77) 11.845 ms 2001:550:3::2ce (2001:550:3::2ce) 57.363 ms 2001:1890:1fff:105:192:205:36:77 (2001:1890:1fff:105:192:205:36:77) 12.125 ms
5 n54ny21crs.ipv6.att.net (2001:1890:ff:ffff:12:122:80:226) 86.448 ms 86.486 ms 84.474 ms
6 cgcil22crs.ipv6.att.net (2001:1890:ff:ffff:12:122:1:2) 84.736 ms 84.832 ms 84.868 ms
7 cr1.cgcil.ip.att.net (
::ffff:12.122.2.53) 86.010 ms 86.177 ms 86.343 ms
8 sffca21crs.ipv6.att.net (2001:1890:ff:ffff:12:122:4:121) 83.847 ms 84.078 ms 83.707 ms
9 cr81.sj2ca.ip.att.net (
::ffff:12.122.1.118) 108.979 ms 108.959 ms 107.975 ms
10 sj2ca401me3.ipv6.att.net (2001:1890:ff:ffff:12:122:126:238) 81.250 ms 81.250 ms 81.379 ms
11 2001:1890:c00:3a00::11fb:8591 (2001:1890:c00:3a00::11fb:8591) 82.225 ms 81.968 ms 82.298 ms