Wiki states that the Engage stage of the spear charge attack should dismount rider mobs.
Steps to Reproduce:
Equip a spear & have some building blocks ready.
Spawn in (encounter) a Zombie Horseman.
Lure the horseman in a body of water to slow it down.
Block off the horseman's path to shore with 2 or more 2 block tall pillars leaving a 1 block gap in between said pillars.
Perform an Engaged Charged Spear attack (mount optional) against the zombie rider.
Expected Results: Zombie rider should dismount/separate from corresponding zombie horse.
Observed Results: Zombie rider remains mounted on the zombie horse, even when both entities took engaged charge attack damage.
Spears require a minimum speed (that varies per material) to dismount mobs. While the current conditions are a little less flexible than they should be (MCPE-235090), the mechanic is working.