I want to add something that many people forget.
There are Hard Enrages and Soft Enrages.

Hard Enrages is whatever you do - you will die on a specific time of moment.
Soft enrages are those that make the fight much more difficult (aka with 2 healers very very difficult) but still survivable.
If you decide to take 3 healers and 1 tank, you make the first mini bosses more difficult and easier the soft enrage.

THerefore this boss is working as intented, it has a soft enrage instead of a hard enrage.