Isle of Chill bug?

Hello, I was recently on Isle of Chill and killed some mobs, at first everything was normal but after some kills I noticed that some mobs spawned but died instantly, and after that they spawned but became invisible and could still attack me!. I could still hit them with an AoE Attack, their names was also not visible then invisible.

And I found that they spawned much faster in that period, it's nice but still not normal.