Why primitives not be clipped in my demo scene on Mali-T860?

Hello,

I've been optimizing Ue4 landscape shader recently.I make a demo scene and it just have a landscape . I found some primitives was clipped when using Ue4 original shader code.Then i try to remove some code about the vertex morph,but the primitives not be clipped in this time.I don't think remove the vertex morph code will make more primitives out of frustum.why the primitives not be clipped?

Here is vertex shader file

  shader.zip

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