If you want to learn more about the enemy's strange movements, try the following.
In edit mode, place a single bar on the bottom floor.
Place the enemy away from the bar.
Place a single object other than a ladder to the right of the bar.
If you place bricks, dig them up.
(Make sure the runner does not fall on the floor.)
Observe the enemy's movements.

Even after playing for years - Enemy behavior is still hard to fully understand.