AP Gangplank's Ult Left Permanent Vision on Dragon

I was utilizing an AP gangplank build, and when I ulted on dragon, the vision the ult gave stayed long after the ult finished. The dragon was not up when I ulted on it, but the vision stayed long enough(3 minutes) to show when dragon was back up. I'll list my items:

1. Crystalline Flash
2. Nashor's Tooth
3. Ionian Boots of Lucidity
4. Rylai's Crystal Scepter
5. Haunting Guise

It was one of those weird games in which the enemy team ALSO had a Gangplank top like me, except that gangplank was ad.