Don't know if it has any relevance to your situation, but a change in anchor would explain your result. For example, one of my anchors is to press the string against the bone behind my eyebrow. For a short bow, my head comes forward more to make this contact, thus reducing the distance between my eye and the arrow, thus reducing my point on. For a longer bow, my head stays more upright, and thus my eye is farther away from the arrow, thus increasing my point on.
When you do something that reduces the point on for a fast bow, it also reduces the midrange gap.