Cause?

I would argue that aspect is more of an issue for VCS, because in kNN you'll always get at least one data point after the first wave passes, whereas in VCS you could get no data even after the first wave passes. Though in both cases you need to handle the no-data case gracefully of course.

What bites me is that I cull points via displacement vectors (points that would lead off of the battlefield), so it's still feasible for me to end up with an empty return till about 1/2 to 3/4 of the way through round one.

I really should drop the displacement culling and see if it hurts/helps. It may be a bit of complexity I don't need.

I record guess factors and displacement vectors for all waves, and come kNN time, I drop data with a (scaled to current situation) displacement vector that leads off the battlefield.

"If it's just for bounds checking, that doesn't make much sense to me in movement." Exactly. Basically, I need to add a boolean argument to that method so that my gun can drop points and my movement doesn't have to. After all, few if any enemies will be doing such culling, so not doing so with movement data will be a closer approximation to the enemy's view of the situation.