Not sure if this is a bug.
Type this:
//window -do @test | timer 1 5 window -u @test
Then select a program other than mirc.
Wait 5 seconds.
The window stays on top until something else is selected.

If you consider this a bug, it would be a windows bug, not a mIRC one. I don't think it's a bug because the zorder hasn't changed so the window has no reason to go behind others.

hrmm ok I would have thought the window would have been put in the background seen it isn't the active window or ontop anymore.