On the English-language Wiipedia site, I've been working through the category "Pages with duplicate reference names" to clean up duplicated reference errors. These errors are caused by two (or more) <ref name="tags with the same name"> that contain different content.
Over the weekend of September 28, 2018, I noticed a large number (about 500) of new entries in the category. Sometimes, this happens because templates that generate named references are edited and start producing duplicated (but different) references. But this time I can't find any such changes that might be to blame.
After asking after the issue, another user suggested this issue is caused by Wikimedia's handling of templatestyles itself, and asked that I report that problem here.
Template:Inflation/fn is one template that seems to be having trouble, and it's talk page has notes about what I've observed. An example effected article is 2010 FIFA World Cup, where subsequent invocations of the template used to work fine (before the weekend) and now generate duplicate reference name errors.
Template:Certification Table Entry is another template with problems, and it's talk page also has notes. The problem with this template seems specific to the use of the Finland region with the "region=Finland" parameter. These issues, too, appeared over the weekend and I can't seem to find any changes to the templates involved in the problem. Affected topics include Call Me Maybe and Jaged Little Pill.
I'm an experienced software developer, but I haven't before reported a bug in Wikimedia's Phabricator; I've got the basics, but I don't know the local customs. If anything is missing or needed, please do let me know and I can do my best to provide it. I'm raising this issue because of the large number of articles affected due to something that changed which seems to interrupt a common-place practice, but doesn't seem to have a cause or remedy available to end-users.