Certain images are downscaled when exporting despite setting to "full size"
Very strange issue I am experiencing exporting JPEGs from my Photos library.
I exported a variety of photos from my library as JPEG - Max Quality - sRGB - Full Size.
While majority exported as expected, certain images (~20%) were downscaled during this process to maximum dimension of 1280px (from 4-5k).
The issue is consistent in that the same images will export correctly, and the same ones will export incorrectly (downscaled) if I repeat the process.
Reverting the image in Photos to the original seems to prevent the issue, so seems related to images that have edits applied (but not all).
Repairing the library did not resolve it.
Possibly related, all the images with an error receive an alphanumeric tag added to the "Special Instructions" ITPC field. This value is constant for any given image but varies between images.
Screenshot of the file metadata attached.
Possibly of interest, all the images with an error receive an alphanumeric tag added to the "Special Instructions" ITPC field, visible in file "Get Info". This value is constant for any given image but varies between images.
I have searched and searched but found nothing quite like this.
For reference and possibly related to the root cause, the library I am exporting from is an old Aperture library migrated to Photos in Catalina.
iMac 21.5" 4K, macOS 10.15