Thanks everyone for the reports - this was a regression following a change I made last week. Here’s a fix:
https://github.com/discourse/discourse/pull/18122
As @denvergeeks worked out, it’s most likely to happen on forums which have the categories+latest (or similar) views as their homepage. On meta it could be reproduced by visiting https://meta.discourse.org/categories in a new tab, then clicking one of the category names.
I’ll update here when it’s merged into tests-passed so that people can upgrade their sites.