Apparently the original switchbar in this topic issue was deemed an intentional change. The expected behaviour is now to highlight the active mIRC window when mIRC itself is not the active application regardless if it is the active *mIRC* window. This is because the assumption is that if mIRC is not active then another window is likely hiding the mIRC window and if it did not go red most users would not know about activity. If you think about it in that sense, the change seems reasonable. Basically, until more people start getting dual monitors or larger screens, this behaviour is likely to stay in order to benefit the majority of users that probably lose sight of mIRC when it goes inactive.
Until then you can use this script to fix the behaviour:
on *:text:*:*:if ($iif($chan,$chan,$target) == $active) window -g0 $v1
