The cause is a browser restriction. If you check your browsers’ error console you will see this report: “Scripts may close only the windows that were opened by it.“. I thought this close button (and the red text to point out the need to use it) solved the problem, and I’m sure it did at the time, but no longer. Hopefully, I will find an alternative, but as you say, it’s not the end of the world. Just manually close the tab and refresh the originating one.
Meanwhile, I will move this to an open bug report.