I'm not entirely sure this is a bug. Yes, DNS in IPv6 mode attempts to resolve hostnames to IPv6 addresses. This is expected.
I completely disagree with you. This is definitely not an expected behavior. /sockopen is not connection dependent and thus shouldn't have different behaviors depending on where it being called from. Currently, /sockopen will resolve based on the mode of the connection it is called from, this can (and in his case does) lead to unpredictable behaviors.
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.
Assuming you have one connection -6 and one -4, /scon -at1 sockopen... will generate an error for the first connection and the other will work perfectly fine. This is something that sockopen should be doing to make sure the dns lookup is the correct one; using /dns is just workaround to this bug, not a solution. The best way to resolve this issue, like you said, is to add a -4/-6 switch to /sockopen AND make it ipv4 dns lookup by default to keep it consistent from connection to connection.
