Support topic: https://www.phpbb.com/community/viewtopic.php?f=466&t=2268416

admin_activate_user.php notification type is assigning usernames to email template variables using user_loader->get_username() in 'no_profile' mode, which in its turn calls get_username_string() function in 'no_profile' mode. This causes HTML markup in emails for username colour, like

<span class="username">XXXXXX</span>