So -a and -d seem contradictory to one another. A desktop window has no "parent", but -a specifically makes it so the currently active window is the parent.
Now, this might not have affected other versions of Windows in the past due to the way the DWM worked with MDI children, but it seems completely understandable that an update to the DWM makes it so any window handle with a parent does not show up in the taskbar.
I would generally consider this expected behavior, since, fundamentally, -a and -d do not really make sense together. In other words, you either want your window to be a root window (desktop), or you want it to be a child, but never both. The workaround is simply to avoid -a if you want a desktop window.
There's also not much mIRC can do here, unless there is a new way to force a child of an MDI window to have a spot in the taskbar, which I would bet isn't possible (though I could be wrong).