I ran into this exact same problem as Jomo, but on my X-T3 recently. 400+ photos from a recent photo shoot were showing up on the camera, but when attempting to read the SD card on the Mac, none of the newer raw/jpeg images showed up for importing via Apple Photos, Image Capture, etc. and I also tried accessing directly on the Finder and via Terminal, but the files were not there either. I also tried to load the SD card on a Windows laptop as well, with the same issues (older photos visible, but none of the new ones). The 400 photos could only be accessed via the camera.
After turning the camera on and off a few times, the 400+ photos from the photo shoot have now completely disappeared from the camera entirely, and I did not have the opportunity to import the photos via the Cam Remote app. My hunch is that the photos were cached on disk somehow, and the cache was wiped out after a few system boots.
Does anyone have any ideas for how to recover these photos?
Given this recent nightmare, I'm thinking I will need to set the card slot setting to "BACKUP" just so I can have photos duplicated on a second SD card, but I don't know if I will run into this caching issue again on both cards.