For some reason my Pages article RSS feed is not in chron order:

https://www.celiac.com/articles.html/?rss=1

It is also showing some older stuff from from Feb. 22, for example, while skipping certain items in between. 

I did just have IPS import all my articles recently from a different application, and we've run the re-indexing on all posts, articles, comments, etc., and this is now complete.

Any ideas?

Double check your database sorting. The RSS feed should be sorted the same way as the database presentation itself. The record 'publish date' is what is used for the pubDate in the RSS feed (so you might, for instance, be sorting the database by last updated and then we display the publish date in the feed, which would make it appear out of order).

Thank you, however, I didn't create the articles CSS, it seems to be built in, and I am having issues finding where in the ACP I would edit it. For example, it is not listed in the [System] RSS feeds. 

The mentioned “database sorting” is set in the Pages database settings. 

Thank you opentype, I found the setting and it updated correctly.