I was fighting the ender dragon with my brother. At one point, possibly due to lag? When we came back to the end after we both died, the dragon was completely invisible. It's model wasn't frozen. (Or if it was, it was out of sight), so this is not the same bug I found on here. It stayed this way until we were both dead at the same time, and then it reset.

I do not know how to replicate this.