I’m guessing this is related to the change in stationary boss aggro, but every time this guy does Flame Lash, his next attack casts onto the same target, killing them instantly.
Went into E7 and his followup normal attacks were doing 16k to the (~7k hp) dpses/healer.
Workaround is tank brings force-aggro abilities, runs them during every Flame Lash.
It also changed so Flame Lash can target the top aggro (tank) not just everyone else.
 this is the first boss in Hell Eternal