I know tasks such as planes do continue to appear even when there is not enough time left to complete them. Tasks that usually allow 2 days, for example, will show a shorter duration if the end of the race is less than those 2 days away. I see no reason to expect mystery tasks to be any different. I like the mystery tasks, though I would only take one is my ships were in, my hot zoo items were ok, and I had plenty of time for the worst case scenario. I find them incredibly tempting, especially as they are coming up a lot in the absence of the event tasks this week, though a few of my co op members refuse to go near them.. as a group, I'd say we do pretty well out of them, getting a horror perhaps 1 time in 4..