How to recreate:
Equip monk with POJ set, Vengeful Wind, and Kyoshiro’s Soul. Movement speed should be 25% from boots/paragon.
Begin channeling TR:Flurry and make fast tight circles with the mouse pointer around the monks feet (use Agility mantra for circle size reference). Flurry stacks are not dropping at this point. Now activate Sweeping Wind and channel TR in the same fashion. When Sweeping Wind stacks reach 13 you will begin to notice Flurry stacks start dropping off frequently.
Seems high movement speed related. You can remove the movement speed from paragon/boots while still having 13 SW stacks and notice it occurs slightly less(maybe half as much).