I'm running LrC (v9.4) on a Mac. I was editing and stacking photos when I started noticing the (!) in the upper right corner on over 40 images. When I click on the ! it cannot find the photo.
Also, many images that had titles are now labeled "Untitled". AND many images have dark borders that indicate they are "stacked", but are next to each other. When I hit ^ , commend, and G, they revert back.
All of this started as I was making COLLECTIONS of photos for calendars.