In regards of usability when learning at max speed:
Pungent sunrise: consumable at max learning speed, successfully decreases learning time by 1 min 30 s (note: you may not observe this if you have lag and close learning window too fast)
Yellow crumb flower: consumable at max learning speed, does not decrease learning time.
Yellow essence: not consumable at max learning speed
I am guessing that only one of those three is expected behavior, and also guessing that is not of the yellow flower :)