I came across this bug yesterday and considering that I’ve just started second legendary armour collection and need to do heart quests, it’s a really annoying bug, making the heart really hard to get. Sometimes (not always though) Cairn channels the spatial attack and no green circles appear. Then the attack goes through, and AFTER the attack (when you’re most likely lifted in the air, unless you luckily stood in an invisible circle or know the position of circles by heart) circles appear for a split second and disappear again.
If I remember correctly, it only happened with greens after the arm sweep attack, the greens that don’t require any minimum number of people to stand in them.