Originally Posted by
gipsymagpie
I have a feeling Prouty talks about that exact topic in one of his articles. I will browse later
So Prouty does talk about the flow around the tail of the AH64 in several chapters of his Helicopter Aerodynamics book. He states "...the much stronger downwash behind the advancing side than behind the reverse flow region...". This would lead to a significant roll attitude in low speed forward flight which would muck up a rocket attack. The gurney flap may also be to remove instability by reducing the chaos and tripping the airflow to separate in a more predictable manner.
But the AH64 tail went through many many variations as they tried to iron out snags (Low stabiliser, T-tail, vibration dampers, slots etc). The flaps are definitely a bodge!