From the description for Circle of Mortality (emphasis mine):
At 1st level, you gain the ability to manipulate the line between life and death. When you would normally roll one or more dice to restore hit points with a spell to a creature at 0 hit points, you instead use the highest number possible for each die.
Two possible scenarios from the linked question:
- You roll for each target. In this scenario, "when you would normally roll" only applies to the single target, since each roll applies to only the single target that roll was for.
- You roll once for the whole spell. In this scenario, "when you would normally roll" applies to every target, since the single roll applies to each target affected, and so replacing the roll with the maximum possible would affect every target.
I do not like option 2. Even if you normally rule spells like mass healing word heal the same amount for everyone, I do not think you should rule this way with Circle of Mortality. My reasoning is that it could result in shenanigans where it's sometimes more beneficial to drop someone to zero hp and then heal everyone from where they are, and that doesn't really make for an immersive experience.