Area effect clouds' position updates (and therefore their Passengers' position updates too) are not sent to the client (in single-player, not tested in SMP), even though, if posessing an actual effect (instead of being a marker/dummy entity), the particles are relocated and displayed at the new position correctly. With hitbox visualization enabled (F3 + B), the white hitbox line and the blue sightline are displayed on the old position, indicating that the client receives only the particle position and the initial position (when spawned or when its chunk is loaded).

Steps to reproduce:

1. {{/summon minecraft:area_effect_cloud ~ ~ ~
   Unknown macro: {Duration}

   }} to summon the area effect cloud (the minecart is there just to visualize the bug, can be replaced with hitbox visualization)

1. /tp @e[type=minecraft:area_effect_cloud] ~ ~40 ~ or a /spreadplayers equivalent to actually move the effect cloud.
2. Notice that the minecart (and the effect cloud's hitbox) is not moved, staying at the initial summon location. Try to punch the minecart, it'll not receive damage (no sound or shaking), although the attack speed meter will indicate that it is punchable.
3. /execute at @p as @e[type=minecraft:area_effect_cloud, distance=..5] run say I'm still nearby! to confirm that the effect cloud (along with the minecart) is server-side at the new location (since it's not detected by the selector that selects all clouds near the player), but isn't so at the client side.