The bug

Attempting to teleport a mob using the caret coordinates will cause them to teleport relative to the direction they last walked towards, rather than the direction they are facing.
Trying to make the zombie/mob teleport forward, it instead moves sideways as that is the direction it last walked even though it is facing a different direction.

/execute as @e[type=minecraft:zombie] at @s run tp @s ^ ^ ^1

This also occurs when trying to teleport other entities relative to the zombie's/mob's rotation.