When one of my creatures in my party takes damage and triggers Stand As One’s damage distribution effect, any minions on other creatures treat this as an enemy hit and react accordingly towards the creature being damaged.
This is most noticeable whenever I’ve picked up some spiderlings from a realm boost - the first one of my creatures to take any damage then causes every other creatures spiderlings to explode at it, often killing it outright. In the rare cases I’ve had an instance of the war minion in the party, it retaliates too. Any minions on the creature actually being damaged act as normal.
Considering that the shared damage of Stand As One doesn’t seem to affect things like the shelled buff, this seems like odd and unintended behaviour, so I’m reporting it as a bug.
EDIT: since first posting this, I’ve been unable to reproduce any effect with the war minion, but with spiderlings it’s trivial, so it might specifically be a spiderlings issue, and what I saw with war earlier was a different interaction.