prop=revisions has long ordered revisions by rev_id rather than timestamp
when returning results for a single page. In most cases this doesn't make a
difference, but for old pages imported from earlier versions of Wikipedia,
for revisions imported from Special:Import, and so on these orderings don't
prop=revisions has now been updated to order by timestamp rather than
id. While this is technically a breaking change, it seems unlikely that any
clients will actually be broken by this.
This should be deployed to WMF wikis with 1.26wmf4, see
for the schedule.
Brad Jorsch (Anomie)