Thats a normal behavior from the aimOffset node. Im not sure why exactly it does that, but thats how it does.
I guess its because it needs to calculate axis based offset delta values for joint transforms which it can only do if there is at least 2 points on each axis.
We would know better if there was a proper documentation on these things -.- … oh well, maybe one day.
So yeah, dont worry, just chose your aimOffset according to your needs (1D or 2D) and populate the axes of that node with at least 2 poses and you should be good.
Remember you also need to specify the additive space in the anim clips used in the aimOffset.