I wonder if this is intentional or what so ever
For who-knows-how-many-days, I have the same stratum types for all my prestige missions (except danger missions).
For all the "Scared to Death", "White Collar Captives" whatsoever missions, I have been having poison for the first depth, and fire for the second. Always!
It's been like that for a while. Although the second depth is freeze sometimes, I think it depends on the time of day.