Steps to reproduce: Cast Taliyah's Q to spawn the initial worked ground, then wait for the cooldown to refresh and cast q again right as you exit the area of the worked ground.
Expected result: Either fires the full volley for 50 mana or fires 1 for 1 mana.
Observed result: Fires full volley for one mana
Reproduction rate: 70% but with practice can easily be done much more consistently.