Hey Aron, thanks for the response!
It's not the SimpleSmoothModifier script, as removing that script causes the exact same issue.
And it's not like the collision just doesn't work, as when adding a Rigidbody2D to the boxes, the boxes are moved by the enemy. The player can also run into the enemy ball and be moved by it. However, the boxes are moved a bit awkwardly, like the seeker ball does not fully respect the boxes' Rigidbody2D settings (moving them too strongly, for instance).
Could it be related to the way that movement is handled by the AI Lerp script? I noticed that it simply sets a new transform.position in Update, probably forgoing the Unity physics engine. I normally use Rigidbody2D.AddForce in FixedUpdate for the movement in the game, as is standard for physics-based Unity projects.
Somehow, I think that the answer lies somewhere in the physics handling.