Spells: Enchanter Faction Spell

[Vandy]

I'm not sure if this is functioning like intended or not.

In Highkeep I have faction of threateningly. When I cast Benevolence on a Guard(Captain Bosec) it will raise it to Dubiously.

I then cast Benevolence on Treasurer Lynn in the back and it resets my faction on the Guards to threateningly.

So is this a bug or are you only allowed to have 1 faction boosted at a time with the spell?