Summary:

Skeleton horseman does not follow the player on skeleton horse.
When the player moves away from the skeleton horseman, they do not follow the player, but simply stand still.

The rest of the skeleton horse mobs follow and attack the player without any problems.

Steps to reproduce:

1. Type the command /summon skeleton_horse ~ ~ ~ minecraft:set_trap.
2. Switch to survival gamemode.
3. Start running away from the skeleton horseman.

Observed results:

Skeleton horseman on skeleton horse will not chase the player.

Expected results:

Skeleton horseman on skeleton horse will chase the player.

Video:

Wither skeleton on skeleton horse
Minecraft 2021-11-06 18-48-26.mp4

Skeleton horseman on skeleton horse (1.17.41)
Minecraft 2021-11-06 19-03-28.mp4

Skeleton horseman on skeleton horse (1.18.0)
Minecraft 2021-11-06 18-44-31.mp4