Custom material changed by pitch?

CesiumJS
1

I use GroundPrimitive to draw a rectangle and set primitive apperance with custom material。

I make a canvas image as fragment texture, here is simplified fragment source :

uniform sampler2D image;

czm_material czm_getMaterial(czm_materialInput materialInput){
     czm_material material = czm_getDefaultMaterial(materialInput);
     vec2 st = materialInput.st;
     vec4 colorImage = texture2D(image,st);
   
     material.diffuse = colorImage.rgb;
     material.alpha = colorImage.a;
     return material;
}

when i change pitch with middle wheel, i found the color of the rectangle change from light green to yellow, so strange.

before

1

after
2

I have tried set Cesium.Appearance property “flat” true, turn off scene.sun, shadows and disable global.lightning, disable fog.

Can anybody know what cause this change . Thank you very muck !!

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