Weird React state behavior

Question

I am working on a React project, and have run into an annoying bug. I have isolated the problem in the code below:

import ReactDOM from "react-dom";
import React, { useState } from "react";

const Variants = ({ variants, setVariants }) => {
  function toggleRemove(index) {
    setVariants((_variants) => {
      let newVariants = [..._variants];
      if (newVariants[index].action === "remove") {
        newVariants[index].action = undefined;
      } else {
        newVariants[index].action = "remove";
      }
      return newVariants;
    });
  }

  return (
    <>
      {variants.map((variant, index) => (
        <div>
          <span>{variant.title}</span>
          <button onClick={() => toggleRemove(index)}>Toggle remove</button>
        </div>
      ))}
    </>
  );
};

const AdminProductTools = ({ product }) => {
  const [variants, setVariants] = useState(product.variants);

  return (
    <div>
      <Variants variants={variants} setVariants={setVariants}></Variants>
      <p>{JSON.stringify(product)}</p>
    </div>
  );
};

const Product = () => {
  const [product, setProduct] = useState({
    variants: [{ title: "dummy1" }, { title: "dummy2" }],
  });

  return <AdminProductTools product={product} />;
};

const App = () => {
  return <Product />;
};

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

When I hit the 'Toggle remove' button next to a variant, I only expect the 'variants' state to change. Somehow the 'product' prop in AdminProductTools is being updated too (as seen in cyan span), even though 'setProduct' is never called. Even weirder is, if you inspect Product in React DevTools, the 'product' state seems to update too, even though it is not reflected in the yellow span.

How can this be?

CodeSandbox: https://codesandbox.io/s/crazy-haze-wk92e?file=/src/index.js