Hmm actually I believe that the 80 Cost Special Mission should not be guaranteed to Drop the 20-100 Manyu's. (Except for the Middle chest that you could miss most runs)

I actually believe that this is a bug that they haven't caught yet probably because nobody is reporting it. So I think that People should just continue to quietly grind out 12-x + 80 Special as long as its available. Unless you need to waste Stamina... Or are in particular need for a certain color of Manyu in particular. I'm majorly Purple short atm I've only maxed my first 5* and nearly done the second but have had to save Purple Manyu's for the newest Event 5* that will be being given out tomorrow.

Given the cost of getting the special garden and it's stamina cost to run I think it is guaranteed