The values of motionX and motionZ are 0 if the player doesn't move in the Y axis.

motionY should also be 0, but is -0.07840000152587890680511151231257827021181583404541015625 instead.

Yes. It's the wrong velocity object entirely.

Pulling this comment from the duplicate, what is the correct place to get the velocity? I was thinking maybe I could make a mixin in Entity that hangs onto the pos before it gets updated later in the same tick, but that didn't seem to work.