I've noticed this too since bosstiary came out. Here's my theory:
During the fight she changes form and seconds after uses death AoE attack.
Notice that when she teleports and is preparing to attack, the sprite of Tenebris changes colors, in this second form if your team do a good amount of damage she will come back to her original form without the death AoE attack.
I can't confirm this yet of course, but it happens when I'm in a team with mostly high levels, they don't usually run but stay to do damage.