What happens if you try
dig -6 www.ibm.com @2a02:8010:1::212:23:3:100

It sounds like a DNS query is timing out to one of those servers and then failing back to the other ones.

I get an instant reply to that query, so I am not experiencing the symptoms that some other posters are seeing, and in some cases only on certain client OSes.

Also by design, a DNS server can return A and AAAA responses regardless of if the query is transported via IPv4 or IPv6.

It seems to me as likely that the problem is with local configuration or an OS quirk than with Zen per se.