I'm not entirely sure this is a bug. Yes, DNS in IPv6 mode attempts to resolve hostnames to IPv6 addresses. This is expected.
It seems that you're asking for mIRC to detect that the bind ip is ipv4 and force ipv4 in this case. I'm not sure it's mIRC's responsibility to do this. If anything, /sockopen needs a -4/6 like /server and /dns has. But this would be an extra feature, not a bug.
Until then you should perform dns yourself on hostnames if you want to enforce ipv4, if you're in ipv6 mode.
