Steps to reproduce the issue

Install Joomla 4.0-dev
Login to admin as your admin
Go to top right and Usermenu -> edit your account -> User Action Logs Options
Email Notifications = Yes
Check all boxes
Save and close
Content -> Articles -> New -> Give it a title -> save and close

Go check your emails.

Expected result

Either a HTML email sent in HTML mode or a plain text email sent in plain text mode with no HTML markup

Actual result

HTML email sent in plain text mode with HTML markup

Full email source:

Content-Type: text/plain; charset=utf-8
Date: Thu, 6 May 2021 17:32:17 +0000
From: joomla4 <phil@phil-taylor.com>
MIME-Version: 1.0
Message-ID: <ezRVdWZWtPaBuioIUwMWsIKIrpWtfoeMjAX5DYxJc@localhost.localdomain>
Received: from localhost.localdomain by mailhog.example (MailHog)
          id ScxpOquk3uhHASxBGlCDwG8y3974VdzRbAq5K6gajOI=@mailhog.example; Thu, 06 May 2021 17:32:17 +0000
Return-Path: <phil@phil-taylor.com>
Subject: Latest User Actions
To: "phil@phil-taylor.com" <phil@phil-taylor.com>

Latest User Actions
------
This is the latest action performed by a user on your website.

 Action | Date | Extension | Name
 User <a href="http://127.0.0.1:4444/administrator/index.php?option=com_users&task=user.edit&id=570">admin</a> added new article <a href="http://127.0.0.1:4444/administrator/index.php?option=com_content&task=article.edit&id=1">erv</a> | 2021-05-06 17:32:17 UTC | Articles | admin

System information (as much as possible)

Additional comments