Thank you for the detailed reply.
I tried deleting the 5 photos taken immediately after the last imported ones, but to no avail. Same problem.
I have not tried optimizing storage yet and I don't have a Time Machine backup (I use clones instead).
I also have not tried creating an new Photos Library, though I did try repairing it. The repair completed OK, but afterward my Photos window now constantly says "Uploading 74 items" but they never seem to upload.
I'm guessing there is some corruption in my Photos Library of some kind and I'll try creating a new library eventually to see if that fixes it. Since I currently have a workaround by logging directly into iCloud to download photos and videos, I'll let it go for now.
