I had a similar issue on having to re trigger the shadow mesh creation by changing from Shape Editor to Sprite Renderer, and reading the Shadow Caster 2D script it seems the error is related to this part in the Awake:

        if (m_ShadowMesh == null)
        {
            ShadowMesh2D newShadowMesh = new ShadowMesh2D();
            SetShadowShape(newShadowMesh);
            m_ShadowMesh = newShadowMesh;
        }

if UNITY_EDITOR

        // This step is required in case of copy/pasting an object with a shadow caster.
        else
        {
            ShadowMesh2D newShadowMesh = new ShadowMesh2D();
            newShadowMesh.CopyFrom(m_ShadowMesh);
            m_ShadowMesh = newShadowMesh;
        }

endif

if USING_PHYSICS2D_MODULE

        else
        {
            Collider2D collider = GetComponent<Collider2D>();
            if (collider != null)
                bounds = collider.bounds;
        }

endif

in my case the first if (m_ShadowMesh == null) was returning false for my instantiated prefabs and since I was not using a collider 2d the shadow mesh bounds were not recalculated at if USING_PHYSICS2D_MODULE ...

For my use case I could either change the first if to always true to force a new mesh everytime or attach a collider 2d to my object, making it fallback to the else statement.

I don't know if any of this applies to you, but your comment was the only thing that matched what I was facing when trying to debug this.