I don't believe this is a bug as it happens to me every fight I have, too :)
If I might explain my take on it:
When you approach an enemy while walking, the names above their head denote their level (Gray less than you,Yellowequal to you, orange higher than you). Once you get into actual combat, ALL the enemies' names, regardless of their level, will show up as Red. This lets you know that they are an enemy. If you notice your pets and summons will show up as Yellow to let you know they are allies.
If you are a Witchdoctor and have the "Charm" Power Card (lets an enemy become an ally for one turn) their name will go from Red to Yellow for that one turn. Then the next turn it goes back to Red.