So just following up here, the photos were not jpegs but HEIF files (thanks @sullivanribbit!) even though they had a .jpg in their file name. Not sure what’s going on with Apple Photos, but that’s beyond the scope of a bug report for iNat so I’m going to close this.
See this feature request about adding support for these files to iNat: https://forum.inaturalist.org/t/support-heic-heif-images/17549/3