Is Vet Depths of Malatar King Narilmor encounter bugged?

Troll92

My issue is this. I know that after he splits the first time, you have to kill the copies at the same time or really fast one after the other while avoiding the original, then you get to DPS him until about 40%(?) and he splits again. Fact is, after we killed the copies, the second we hit the original, he just split up again without us getting a chance to do any damage, then after we killed the red copy by mistake a bit faster, he split in 3 more. The fast we didn't get to DPS him at all after killing the first 3 copies, is this a bug? Or does he split faster and more often on vet?