I have a couple Vodyani fleets orbiting an enemy system (A Harvesting Fleet and an Attacker Fleet). I set my Attacker Fleet to be on Guard while my Harvesting Fleet is intentionally not set to Guard. However after my Attacker fleet is attacked by an enemy, next turn they won't be set on Guard and my Harvesting Fleet will be. 

Is this a bug? If not i think this mechanic should be changed to how it was in ES1.

Edit: Ok, played around with it a bit more and it seems like you can't just have 1 particular fleet guarding without all the fleets orbiting the same system guarding as well.