"Export as catalog" missing photos

I have a strange problem with Lightroom Classic. I have all my photos on one catalog. As part of my backup routine, i will export my photos by year to a drive that i keep out of my house. Since my folders are organized by year, this is very simple. I just right-click on a year folder and choose "Export as catalog". This creates an lrcat file and a folder with every image file from that year.

Up until now, this has worked perfectly every time. However, when i try to export my 2017 folder, i get the error message "Photos could not be found". I have tried four times with slightly different results each time. Out of the 11500 photos, between 4500 and 5500 cannot be found. My initial thought was that Lightroom had somehow lost track of the photo files, but in Lightroom, these photos are just fine.

I have also tried to export my 2018 folder and that worked perfectly.

Any suggestions on how to solve this?

(I have a three level backup routine that i'm very confident with, so that's not something i seek to discuss in this thread unless someone is looking for tips on how to design a backup routine.)