That's one of the differences between titleCut (which - obsolete or not - still works fine for me) and titleText. That third parameter doesn't work with titleText, at least OFP ignores it.
To make titleText disappear before it's time (here: after one second):
titleText ["Quest for Osama", "PLAIN DOWN"]
~1
titleText ["", "PLAIN"]
This way there is no fade out effect though.