We see the same sudden increase in CLS failures for meta.discourse.org, starting 9th November. Digging in, it seems to be related to the way we animate the background-color of posts when you navigate into a topic. That behaviour has been the same for 5+ years, so that suggests to me that this is a change to google/chrome’s measurement of CLS, rather than a change on the Discourse side.
We’ve decided to drop the animation in question. That should improve navigation performance on lower-spec devices, as well as resolve the CLS issue.
That change will land on Meta imminently, and I’ll get it queued up for your site as well @npearson99.