I haven't seen this reported but it's hard to imagine I'm the only one experiencing it...

Whenever I use the Ctrl+F Find Text window, I have to repeatedly click "Up" to get to new lines. It will jump back down to ones I have already seen. It can be very frustrating. If needed I can make a video to show an example of what I mean. As far as I can tell this only happens when searching Up.

Not 100% sure, but..

It MAY have to do with new text being put into the channel, which could be making mIRC think you haven't actually seen the last line that you saw.