[fadetree]

AC is composed of two values : mitigation and avoidance. It shows as one number but internally it is handled separately. AGI affects only the avoidance component, so increasing AGI will make your displayed number go up, but the question is how the game actually handles avoidance internally. There's some real questions here - the overall effect, or lack of it, (or the possibility that it's actually negative on P99) might be hard to directly notice so there's no way to tell without some rigorous old school parsing.
Getting in the wayback machine and looking at old material from Ranger's Glade, Steel Warriors, and The Safehouse might provide some context, but of course we'd need to parse P99.