That may happen if you have overriden the /join command with a poorly written alias, for example
/join /join #$$1-
and you have enabled the option "Minimize on join" for a channel in Favorites. mirc executes internally "/join -n <selected channel>". This calls the above alias, which prefixes -n with #, thus making mirc join #-n.
I suggest you avoid aliasing mirc commands or at least do it carefully.
