I have 97 JPEG source files. I ran batch jobs to apply crops to these and save the outputs as JPEG to another folder. For three runs of this, I had the following results:
All 97 files processed okay
1 file was skipped
3 files were skipped
By skipped, I mean that the affected file simply did not appear in the output folder. The files skipped appeared random. They were not at the beginning or at the end of the list. There were no error messages. I manually redid the skipped files without problems. The input files all had identical dimensions of 600 x 1800 pixels. The batch macros consisted of one crop only -- no other changes.
MacOS 10.13.6; iMac (21.5-inch, Mid 2011) 2.7 GHz Intel Core i5; 12 GB RAM; Affinity Photo 1.6.7
I would be willing to provide info or test cases to reproduce this.