Tibbers not moving

So I was playing a game as Thresh. My girlfriend was playing Annie. It was just a bot game, so I went Thresh mid (ADC build) and decided to start roaming around in the Jungle. I came Top where my girlfriend was, and she was running off an enemy bot towards his Inhibitor Turret. I was busy taking their Krugs watching her play and letting her get the kill. I watch her summon Tibbers to cut off the enemy. Tibbers doesn't move. She summoned him in the threshold of the jungle, and he stayed there- not moving. (I told her to back off a good distance to see if he teleported to her side, but she was in the zone and said f*&^ it). (I ended up chasing for the kill- not on my watch- not on bots) But- it was kind of upsetting, seeing as she did really well against the bot (she's new), and relied on Tibbers for the finish.. but he bugged out.

So- the specifics: Casting Tibbers in the first Jungle threshold (top-krugs side) will render Tibbers immobile.