According to this question, Invisible, but unhidden enemies are sensed automatically. However, this seems to contradict the Ranger's Feral Senses ability.
FERAL SENSES: You are also aware of the location of any invisible creature within 30 feet of you, provided that the creature isn’t hidden from you and you aren’t blinded or deafened.
This suggests that being able to sense unhidden invisible enemies is something special that only rangers can do. Is there a consistent ruling under RAW?