The patchnote said "Fixed Clear History bug that incorrectly cleared channels list settings."
I believe it's the channel buffer, where you can save an amount of lines. Which means when you connect again, you can see previous conversations from the last login(s)in the same channel window.
Now when I used /clear before the mentioned patch, for this one channel, the setting was "cleared" as well.
Meaning connecting with mirc, I'd see the prior conversation from all but one channel.