If you take a look at the Rigidbody2D component in the inspector and open the "Info" foldout you'll see that the body isn't rotating at all. What is happening is that the Quaternion for rotation is providing different solutions to the same rotation causing issues (flipped Quaternion). This isn't dealt with in the physics system.

Inherently this is a 2D system so rotating in 3D space makes it difficult for a system that only has a single degree of rotational freedom (Z).

It is highly recommended that you use Transform.localScale to flip the direction and NOT rotate in 3D space as you can encounter these Quaternion ambiguities.

Here's the change:

public void Flip()
{
#if false
transform.rotation = Quaternion.LookRotation(-transform.forward, transform.up);
#else
transform.localScale = Vector3.Scale(transform.localScale, new Vector3(-1f, 1f, 1f));
#endif
}

With the above change, the Transform quaternion doesn't alternate and cause issues. This issue is one fo the few times this has come up so making major changes in an attempt to deal with this isn't a risk we can take right now.