Were you shooting a springer?

They tend to shoot high and to the side prone.

Basically, springers need, as you know, a loose hold, and prone tends to be a strong hold. Takes real effort to loosen it up. Counter-instinctual.

RF/CF this doesn't really apply. Though I have found that they shoot to a slightly different POI between prone (with a tight sling) and kneeling or offhand.

PCP I have no idea. Never owned one.