As per the customer, the issue was specific to a document and rebuilding the master page items solved the issue.
Adobe InDesign TeamGG commented
As I continued to work on this document, I noticed that if I apply an override to items on the Right page in the spread first and then apply an override to the Left page of the spread the issue does not occur as long as I make the edits to the Left page before moving on to another page. If I move to the next page and then come back to the previous page and edit the Left page objects that were overridden, the original placeholder text remains in the background.GG commented
I have similar issue with overrides on a Left page of a spread. When I Ctrl+Shift click a master text block to edit it, the block becomes editable, but the placeholder text still appears in the background. If I immediately move the text block up or down, the original placeholder master text block disappears. If I do something else then try and move the text box, the original remains. However, I saved the document, closed it and reopened it and the original master text was gone. (Using CC ID 14.0.1 on an iMac with Mojave)GG supported this idea ·