Make sure you didn’t accidentally switch to the non-federated view (you can find this federation toggle if you click the triangle icon next to the UI settings icon).
I definitely have the federation option on, I've even looked in a different browser just in case the setting was stuck off and was glitched, and the newest sort still only has articles from kbin.social or occasionally a federated magazine but from a kbin.social user. Do you see a lot of posts from other instances in your newset tab?
I had to scroll a bit but I did come across some (1-2 hr old) posts from Beehaw and the such eventually. It did seem a bit harder to find these with the newest tab as opposed to the other sorting options, but that might just be coincidence.