See for instance https://commons.wikimedia.org/w/index.php?title=Special:AbuseFilter/examine/1200691612. This happens since 1.33-wmf.22, and is caused by https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/AbuseFilter/+/445325/. Probably because that patch stayed on gerrit for a long time and the changes to T173889 weren't applied. I still have to determine the impact, but it could be only visual. At any rate, I'm coming with a fix shortly.
Aye, I was right! The prefix 'ARTICLE' wasn't updated to 'page'. The impact is that, in order to test uploads, users have to use article_* vars instead of page_*. Nothing else is affected and no need to backport as these values aren't saved in the DB.