This was my reply from Canvas support on 1/15/22

This is Alan with L2 support. We have received your ticket in regards to the screenreader-only class that is added to pages and hiding some text. You are wondering how that is showing up. That will depend on the process of how the content is being added to the pages. Are they copy/pasting content in to the page when editing? A lot of the formatting will come along with content that is copy/pasted from other locations. If that is the process, they would need to watch for that class and delete that from the html after they have posted to the content editor. If you have any other trouble or questions, let us know anytime.