It's neither instance's or the recent event's fault. It's been that way for a while now and is just a Lemmy problem that is being worked on. It has improved since the last month though so you're probably seeing the older posts that didn't replicate at the time. It was basically giving up too quickly when looking for posts from other instances which caused a lot of mismatch between what local communities showed and what it looked like on federated instances.
More details here: https://github.com/LemmyNet/lemmy/issues/3101