Disabling fieldsets does not work as expected

In my demo form, fieldsets are enabled sequentially on button selection. So checking a button in first fieldset enables the second, and so on.

If all checkboxes are unchecked in the first fieldset, the form should reset and then second to fourth fieldset should be disabled again. But this is not what happens. Second fieldset somehow remains enabled. What is the problem here?

document.addEventListener("DOMContentLoaded", _ => {
  const form = document.forms[0]

  // First, disable all fieldsets except the first

  function disableFieldsets() {
    const disabledFieldsets = form.querySelectorAll(
      "section:not(:first-of-type) fieldset"
    )

    for (let i = 0; i < disabledFieldsets.length; i++) {
      disabledFieldsets[i].disabled = true
    }
  }

  disableFieldsets()

  // Sequentially enable fieldsets on selection

  const sections = form.querySelectorAll("section")

  sections.forEach(section => {
    section.addEventListener("change", function() {
      let nextFieldset = this.nextElementSibling.querySelector("fieldset")

      if (nextFieldset) {
        nextFieldset.disabled = false
      }
    })
  })

  // Reset form and disable fieldsets after all inputs in the first fieldset are deselected

  const firstFieldsetButtons = form.querySelectorAll("[name=First]")
  let isChecked = false

  firstFieldsetButtons.forEach(firstFieldsetButton => {
    firstFieldsetButton.addEventListener("click", function(e) {
      if (this.checked) {
        isChecked = true
      } else {
        form.reset()
        disableFieldsets()
      }
    })
  })
})

<form method=post action=submit>    
  <section>
  <fieldset>
    <legend>First Fieldset</legend>
    <label><input type=checkbox name=First value=A>A</label>
    <label><input type=checkbox name=First value=B>B</label>
    <label><input type=checkbox name=First value=C>C</label>
  </fieldset>
  </section>
  
  <section>
  <fieldset>
    <legend>Second Fieldset</legend>
    <label><input type=radio name=Second value=1.1>1.1</label>
    <label><input type=radio name=Second value=1.2>1.2</label>
    <label><input type=radio name=Second value=1.3>1.3</label>
  </fieldset>
  </section>
  
  <section>
  <fieldset>
    <legend>Third Fieldset</legend>
    <label><input type=radio name=Third value=2.1>2.1</label>
    <label><input type=radio name=Third value=2.2>2.2</label>
    <label><input type=radio name=Third value=2.3>2.3</label>
  </fieldset>
  </section>
  
  <section>
  <fieldset>
    <legend>Fourth Fieldset</legend>
    <label><input type=radio name=Fourth value=3.1>3.1</label>
    <label><input type=radio name=Fourth value=3.2>3.2</label>
    <label><input type=radio name=Fourth value=3.3>3.3</label>
  </fieldset>
  </section>
  
  <input type=submit value=Submit>
</form>