I can't seem to reproduce this issue here. I install a new copy of mIRC 6.21 into a clean folder, and then connect to an efnet server. I then go here
and click on the Undernet link. The links dialog pops up showing the efnet server as the currently connected to server, and the undernet server as the server to connect to.
Are you able to describe a step by step process that reproduces this with a clean install of mIRC 6.21?