IGN: Tokari Issue: Snow Rose isn't dropped once the May Mist has spawned the reactor. The reactor takes about 15-20 hits to be killed and there is no drop from any of the 5. This effects the completion of "A Rose from the Snowfield" and Humanoid A wants to be Human. According to today's update, this should have been implemented. However it is still broken.
You are not guaranteed to get one in 5 tries, it should most definitely drop as it's in the drop table.
Then what is roughly the drop-rate? BBB has no info on the drop-rate, and basically alludes at it being dropped by 1 of the 5. http://bbb.hidden-street.net/quest/mu-lung-nihal-desert/humanoid-just-wants-to-be-human