I'm right there with you!
I just now upgraded my Mac to the new OS ("Sonoma") and went back to look at the Z8 photos that I tried importing into the "Photos" app in July. They are STILL appearing as blank, gray squares!
As an experiment, I then tried importing a few hundred Z8 photos that I stored in a folder on the external drive where I keep my Mac's "Birds & Wildlife" photolibrary. And guess what? As all of those Z8 photos imported, the proper, actual thumbnails of each photo appeared on the screen! For about TWO SECONDS! And then the photos app sequentially removed each image, in rapid fire succession, and replaced it with a generic, gray box!!!
WHAT THE ... ??