Update:
Thanks to KindOne and Raccoon, for helping on this issue
https://forums.mirc.com/ubbthreads.php/topics/264163/channel-notices-in-other-channels

I tried searching for this topic before posting, but the above thread didn't show up for me, and I didn't remember having posted in it.

Since I didn't have ChanServ in more than 1 channel, I didn't realize that the behavior of notice was even more unexpected, where if your active window isn't a channel shared with Chanserv, or whoever is the nick sending the notice, the behavior is to repeat the notice across every shared channel window even though only the 1st window is highlighted in the switchbar.

I'm thinking that what people are expecting from chanserv notices is for the behavior to be consistent across all networks, and that generally means that the notices always go to the status window

A hack that makes the issue to away until the next reboot, or until the next time chanserv 'joins' a channel, is to deceive mIRC into thinking that replies from /chanserv help are no longer coming from someone in your nicklist

//if ($comchan(chanserv,1)) parseline -iqtn : $+ $address(ChanServ,5) NICK :ChanservFake