This is caused by a browser caching issue with the Course Summary tool. When an instructor clicks on Course Summary link on a given course, the ID for the summary is loaded into the browser's cache. If an instructor has multiple Course Summaries for different courses open in different tabs in the browser, the browser is going to remember whichever ID was the last to be loaded. Any changes made on will show up in the summary text of the most recently loaded course, and not the course they're working in.
This issue may affect any instructor who is working on multiple Canvas courses in the same browser.
This issue is present as of 8/23/2019.
Instructors can avoid this issue by working on one course at a time.
However, if they have multiple courses open in the same browser, they can prevent the issue by making sure to click the Course Summary navigation link in the course they are trying to edit immediately before making edits.