Every time I generate metadata for Archlinux results in some 10/15 icons missing from the exported data. The missing icons are different every time, so it doesn't seem to be a problem of any specific package but rather some kind of race condition. There are no visible errors in the asgen output.

Attached is the used asgen-config.json. Best way to reproduce it is to run the generator a few times in a row, and do a diff on the generated icon tarballs.
asgen-config.json.txt