Sorry I forgot to take screenshots, but this happened to me twice today.
First with the Wengari Patriach who teleported to above some pond and his character graphic started flashing. Although I was using a ranged attack but I couldn’t damage him whenever he “flashed” to invisible.
Second time this happened with the Primeval Dragon, who teleported off screen and became invisible, and I could only guess where it was by the flames it spewed. Both times I died from attacks coming out of nowhere. This is the first time I have experienced this and I never encountered this before the latest patch.