[GAMEPLAY] Sundra Ult stopped by Ezreal blink

Syndra's ult is stopped by Ezreal using his blink (not flash) to escape my team's sight range. I thought Syndra's ult was unstoppable and has been in the past. Happened both times in my last game when I ulted Ezreal. He was at low health and my ult would have killed him. I had 3 balls left to be thrown and as soon as he left my sight my ult stopped.