Subadult Triceratops floats into sky when attacked while sleeping at Salt Flats

I played as Megalania on Salt Flats. I attacked a sleeping subadult Triceratops. It stood up to fight, then froze and floated into the sky. I couldn't hit it after it floated up. This happened 9 minutes before server restart.After it floats into the sky, my venom effect fades away gradually.

Subadult Triceratops that was asleep stood up to fight back after being attacked, then suddenly froze and floated into the sky, and none of my attacks could hit it.After it floats into the sky, my venom effect fades away gradually.

The Triceratops fights back against my attacks.