Holding and bracing and balancing a 10-20 foot spear while moving sideways or backwards in particular, it's not going to be stable, doesn't matter if you're a powerlifter or a malnourished peasant. There's a reason why in combat sports, knockouts when moving backwards are seen as such a high end skill, because it's really difficult to generate power and strength while moving on the back foot. Hence why I think it should do max damage when stationary and considerably less when moving, to account for the huge loss in stability. It'd likely still be enough for unarmored horses but it certainly shouldn't do much against an armored war horse. I doubt we'll see Taleworlds put in AI that makes horses stop on their own.
That being said, I know in Warband there was a mod where you could root a shield into the ground so that you could reload crossbows from behind cover. Perhaps we'll get a mod in Bannerlord where you can brace your spear into the ground.
Other things to consider are that these long weapons should often break under the impact of a 1000+ lb horse. And the person holding it should often get knocked down by the impact as well.