Tested rebirth and soulstone so far, both bugged.
They ignore the 20% mana rule and just bring people back with whatever mana they had before they died.
Same happened twice last night during our raid. If this is a undocumented feature (aka bug), or a genuinely cool implementation from blizzard, we'll probably notice soon. Well, servers just restarted actually, it could have been to address patch bugs, we'll see soon enough.