This sounds more like a Feature suggestion rather than a bug.
Since you regularly want to connect to these servers, my recommendation is to use a small script that runs when you start mIRC.
The basics for the script have been posted many times before, and basically look like this
on *:start:{
.server irc.network1.com -j#chan1,#chan2,#chan3
.server -m irc.network2.com -j#chan1,#chan2,#chan3,#chan4
.server -m irc.network3.org -j#chan3,#chan4
}
Of course, you'll have to replace the network and channel information to make it appropriate for your situation.
I also recommend, if you do it this way, that you disable the Enable perform on connect, which is located at mIRC Options -> Connect -> Options -> Perform
You can get more details about the /server command and the various switches and parameters (I kept it simple in the code), by looking at /help /server