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 always match.
prop=revisions has now been updated[1] 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 https://www.mediawiki.org/wiki/MediaWiki_1.26/Roadmap for the schedule.