Describe the bug
Hi, I uses UIParticle.cs to control my custom particle system on UI.
I write a custom shader which uses "custom vertex streams" to control like "uv flow" of texture by artists.
But I found it looks like only TEXCOORD0.xyzw will work well.
If I use TEXCOORD1, TEXCOORD2.... , the values fetched in shader not correct.
I have tried to use the same setting (with same particle system and same shader), make particle system not controlled by "UIParticle.cs", it will be correct.

I am not sure whether it's limitation for UIParticle.cs or maybe just I didn't setup correctly?

Like the image below, I use Custom1.xyzw (TEXCOORD0.zw/xy) as custom value.
It will use
custom1.xy => represent TEXCOORD0.zw in shader.
custom1.zw => represent TEXCOORD1.xy in shader.
This is how I get particle system input value in shader, in this case, TEXCOORD0.zw are correct, but TEXCOORD1.xy are always wrong....

I have no idea what happened....

To Reproduce
Steps to reproduce the behavior:

1. Enable "Custom Vertex Stream" of renderer of particle system.
2. Add Custom1.xyzw (TEXCOORD0.zw|xy)
3. Implement custom shader and declare attribute like the "offset" below to fetch custom data input on particle system:

        struct Attributes
        {
            float4 positionOS   : POSITION;
            half4 texcoord     : TEXCOORD0;
            half2 offset : TEXCOORD1;
            half4 color        : COLOR;
        };

4. Make texture "offset" in shader like :

Varyings Vertex(Attributes input)
{
	Varyings output = (Varyings)0;
	output.positionCS = TransformObjectToHClip(input.positionOS.xyz);
	output.uv = input.texcoord.xy + input.offset;
	return output;
}

half4 Fragment(Varyings input) : SV_TARGET
{
	return SAMPLE_TEXTURE2D(_BaseMap, sampler_BaseMap, input.uv);
}

Expected behavior
If you feed "offset" value by "custom data" on particle system, the texture should be "offset".

Environment (please complete the following information):

• Version 3.3.9
• Platform: Standalone(Windows)
• Unity version: 2020.3.14
• Build options: Not built, editor can reproduce it.