Should they be landing on their feet when they're de-horsed?
Of course they should.
No, what I meant is that I don't see the need for such a tremendous difference in the duration of the nap on the ground between :
weapon knockdown or full horse bump
and
dehorsed rider.
In the first case, it is very frequent for people to block while they are still in the stand up animation, which is only allowed much later in the second case.