May I ask how this flash jump in the 2nd job work? Do I need to learn it to get the triple jump? And is this triple jump vanilla?

Sap of Ancient Tree really is that rare from Dark Axe Stumps (about one in every 170; Stumpy drops it far more often), but you're right that the reward didn't match the work. Sophia's Snack now gives 19,000 EXP. About Flash Jump: the 2nd-job Flash Jump is optional and gives one extra jump. The Hermit's Flash Jump at level 20 gives the triple jump, and that one is our own addition, not vanilla.