The first form is missing a closing </form> tag. As a result, all of the inputs that follow it in the document are included in handle submit event. To fix this we can just add the closing tag and each form will submit independently.

<main>
  <form onsubmit="handleSubmit(event)">
    <fieldset>
      <label for="name">Name:</label>
      <input type="text" name="name" value="Tyler" />
      <label for="age">Age:</label>
      <input type="number" name="age" value="33" />
      <button>Submit</button>
    </fieldset>
  </form>

  <hr />

  <form onsubmit="handleSubmit(event)">
    <fieldset>
      <label for="company">Company:</label>
      <input type="text" name="company" value="ui.dev" />
      <label for="employees">Employees:</label>
      <input type="number" name="employees" value="33" />
      <button>Submit</button>
    </fieldset>
  </form>

  <script>
    function handleSubmit(event) {
      event.preventDefault();
      const formData = new FormData(event.target);
      alert([...formData.entries()]);
    }
  </script>
</main>