I don’t know if anyone else has seen this, but after the InDesign update this morning (16 June) to 15.1, all the current page numbers on documents are the letters “ss” instead of the page number. It appears on all pages, whether the “current page number” marker is on a master page or put on the page individually
Okay, I asked on the other boards, and it turns out it’s a bug in the World Ready Paragraph Composer. I’ve gone back to InDesign 15.0.3 so that I can get some work done.