Hi Dan - thanks for the welcome!
I tried your suggestion (pretty sure I tried it before) - still no go. When I double click an .afdesign file, it opens in Photo. I also have folders with identical icons in my iCloud Drive for both designer and Photo. Things are weird. (see image)
gregspiral - thanks for your help. I've been doing exactly the same procedure that you show in your videos. It was one file, started on the iPad, then edited extensively on my mac, that was failing to sync. I ended up closing it on the iPad (from within Designer), then importing the new file from my iCloud drive. Worked fine, and has been syncing properly since. Mysterious and a bit unsettling.