When fighting a Demon training boss, I have encountered issues trying to heal people.
The first time I was in faction mode and couldn't heal a person of another faction even though we were in the same fight. I can see how this couldn't be considered a bug, but we were working toward the same goal.
I've had issues healing other people who were in faction mode when I wasn't in faction mode but I didn't pay close enough attention to see if we were in opposing factions until yesterday.
1) I was not in faction mode (bard, mysticism)
2) Two people in my team were in faction mode (reaper and either a berserker or spearmaster)
3) we were in the same faction
4) I could heal them intermittently but not the entire length of the fight. I was a bard so i tried spamming both group heal and the direct heal.
I am not sure what rules the game is supposed to follow if people in opposing faction modes join the same party, but I checked and both of them were in the same faction as i was.