Same issue here, starting at some point this week. I had made no changes to my site and tested clearing Custom CSS to confirm that wasn't the culprit. I've reported this to Squarespace and am waiting to hear back but would appreciate any suggested fix or workaround. For clarity, this is with 7.1 using Portfolio Pages. Layout is set to Hover Background and width is set to Full Bleed. This worked fine for a year (hovering over a portfolio title would change the full page background) but now the background doesn't change behind the site header.