Niagara Partcle Unique ID is not Unique

Hi all, I’ve written a scratch module in my niagara system:


I thought it means 1500 particle will cover all 1500 vertices of my mesh, but it didn’t. Some of my vertices are not covered at all, and some have multiple vertices covered it.
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I tried enabling Persistent ID and switching CPU/GPU, but nothing helps.
It seems like Unique ID is not unique at all.