This was due to an ordering issue in META-INF/INDEX.LIST. The file is
generated before running strip-jar-timestamps, so the order of the
content is not deterministic when the file is generated. In
79d4d47b99591e4edd8cefee907fbb0d930f78aa I added a phase before that,
that runs strip-jar-timestamps before and after generating INDEX.LIST
(after because INDEX.LIST otherwise has a timestamp).

The fix was pushed to staging.