This is easy to reproduce, type:

/echo #main,#chat

Now, move the mouse over the "#chat" part of that, you'd expect to join #chat, however mIRC joins #main. The problem is obviously due to the comma. Imho, it should work one of two ways, either join both #main and #chat, or it should detect "is the mouse over #main or is it over #chat?" it shouldn't always join #main.

Yep... I think mIRC should conform with how it handles other channel lists and only join the channel the mouse is over.

Of course adding a space makes it a non-issue.

you can only (practically, not exclusively) add a space if you entered the string, as in codemastr's example. however, in a more realistic situation, when someone else typed that in a channel or query, you could not just insert a space, and therefore are relying on the way mirc handles the hotlink.