It seems mIRC does remember that the 2nd tab has the focus, but just doesn't do anything else about it. I tried your example, and the 2nd tab did not only not get focus, but clicking on it had no effect either: I couldn't select the 2nd tab untill I clicked on the 1st tab. Pressing [->] only had efect after pressing [<-] first. The 1st tab also flickered a little when I first clicked it/pushed [<-].
