Can't reproduce this. Please list your mIRC version, OS, and any other relevant info (how you are connected to IPv6, etc).

Code:
/dns 2001:470:1f11:6b5::2
* Dns resolving 2001:470:1f11:6b5::2
-
* Dns resolved 2001:470:1f11:6b5::2 to ipv6.servxtreme.info
-


And see if /dns -6 2001:470:1f11:6b5::2 works