MutationObserver script for Tampermonkey works in Firefox, but not in Chrome

Here's the script that saves the state of the elements as they appear (saveMessage function), and then uses MutationObserver to monitor the changes in the className of an element and to restore it's saved state (resetMessage function). Actually it was working in Firefox until one of the recent patches (v.104 I believe) and wasn't working in Chrome for at last a year (the text of the elements with className 'chat-line__message--deleted-notice' won't be restored, just nothing happens). Now it just does seemingly nothing in FF, but because it was working in FF and not working in Chrome at the same time, I assume it must be some very small deviation from the standard or something like that. Any ideas what's wrong with it?

// ==UserScript==
// @name         Twitch: <message deleted>
// @namespace    https://greasyfork.org/users/221926
// @version      1.3
// @description  show deleted messages in twitch chat
// @match      https://www.twitch.tv/*
// @grant        none
// @run-at       document-end
// ==/UserScript==

(function () {
  'use strict'

  function saveMessage (el) {
    if (el.initialChildNodes) return
    el.initialChildNodes = Array.from(el.childNodes)
  }

  function resetMessage (el) {
    while (el.initialChildNodes == null) { el = el.parentNode }
    while (el.lastChild) { el.removeChild(el.lastChild) }
    el.initialChildNodes.forEach(childNode => el.appendChild(childNode))
    el = el.closest('.chat-line__message')
    el.style.backgroundColor = 'rgba(255, 0, 0, .1)'
    el.style.boxShadow = 'inset 4px 0 0 0 rgba(255, 0, 0, .4)'
  }

  new MutationObserver(mutationList => {
    mutationList.forEach(mutation => {
      Array.from(mutation.addedNodes).forEach(el => {
        switch (el.className) {
          case 'chat-line__message': saveMessage(el.querySelector('.chat-line__username-container').parentNode); break
          case 'chat-line__message--deleted-notice': resetMessage(el); break
        }
      })
    })
  }).observe(document.body, { childList: true, subtree: true, characterData: true })
})()