Kinah/Charlie
It's nothing medical, that's for certain. We've come a long way in terms of dampening or suppressing powers, even altering them a bit, but erasing the genetics entirely? Not possible.
It's also not psychic influence, since that would simply make the powers inaccessible mentally.
It could be magic, because magic doesn't operate on the same levels, but that sort of magic would have to be ancient, powerful, and incredibly complicated.
If we're being honest, the most logical possibility in my mind is the one that we go to when we run out of options: reality warping. However, that's a last-resort explanation, given its rarity.
And it ain't like isn't like the data's conclusive. Astrid's an anomaly, but I have appointments with others affected later in the day. Bex Lebeau also lost her powers, so we'll see if that lines up.