Setting MaterialPropertyBlocks breaks batching

I have a prefab with the CircleCollider and following script attached:

 using UnityEngine;
 using System.Collections;
 
 public class ShakeMaterialOnTrigger : MonoBehaviour {
 
     public float shakeTime = 0.2f;
     public float amplitude = 0.1f;
     public int oscillations = 3;
 
     private static int offsetID = -1;
 
     void Awake () {
         if (offsetID == -1) offsetID = Shader.PropertyToID("_Offset");
     }
 
     void OnTriggerEnter2D (Collider2D collider) {
         StartCoroutine("Shake");
     }
 
     IEnumerator Shake () {
         float t = 0;
         while (t < shakeTime) {
             t += Time.deltaTime;
             float frac = Mathf.Clamp01(t / shakeTime);
             float x = Mathf.Sin(frac * oscillations * 2 * Mathf.PI) * amplitude * (1-frac);
             SetOffset(new Vector4(x,0,0,1));
             yield return new WaitForEndOfFrame();
         }
         SetOffset(new Vector4(0,0,0,1));
     }
 
     public void SetOffset (Vector4 offset) {
         MaterialPropertyBlock props = new MaterialPropertyBlock();
         renderer.GetPropertyBlock(props);
         props.AddVector(offsetID, offset);
         renderer.SetPropertyBlock(props);
     }
 }
 

It also has a material using the following shader:

 Shader "Custom/Offset Verts" {
   Properties {
     [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
     _Offset ("Vertex Offset (X, Y, Angle, Size)", Vector) = (0,0,0,1)
   }
 
   SubShader {
     Tags { 
       "Queue"="Transparent" 
       "IgnoreProjector"="True" 
       "RenderType"="Transparent" 
       "PreviewType"="Plane"
       "CanUseSpriteAtlas"="True"
     }
 
     Cull Off
     Lighting Off
     ZWrite Off
     Fog { Mode Off }
     Blend SrcAlpha OneMinusSrcAlpha
 
     Pass {
       CGPROGRAM
       #pragma vertex vert
       #pragma fragment frag
       #include "UnityCG.cginc"
       
       struct appdata_t {
         float4 vertex   : POSITION;
         float4 color    : COLOR;
         float2 texcoord : TEXCOORD0;
       };
 
       struct v2f {
         float4 vertex   : SV_POSITION;
         fixed4 color    : COLOR;
         half2 texcoord  : TEXCOORD0;
       };
       
       float4 _Offset;
 
       v2f vert(appdata_t IN) {
         v2f OUT;
         OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
 
         float ang = radians(_Offset.z);
         float2 pos = OUT.vertex.xy * _Offset.w;
         pos = float2(pos.x*cos(ang) - pos.y*sin(ang), pos.x*sin(ang) + pos.y*cos(ang));
         pos += _Offset.xy;
         OUT.vertex.xy = pos;
 
         OUT.texcoord = IN.texcoord;
         OUT.color = IN.color;
         return OUT;
       }
 
       sampler2D _MainTex;
 
       fixed4 frag(v2f IN) : SV_Target {
         fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
         return c;
       }
       ENDCG
     }
   }
 }
 

There are a few hundred of these in the scene. They're all marked as static. When I hit play, the draw call sits at 2 (one for the character, one for all of the grass), saved by batching is at a few hundred. The effect I'm looking for is there (grass shakes as you walk through it), but each time another object gets triggered, draw calls go up, batch count goes down. Am I misunderstanding something about how MaterialPropertyBlocks work? For completeness, I've also tried clearing the block before setting it.