I am seeing the same thing. I saw in another post that you can only see content from other communities after someone from your home instance has subscribed to it.
I created a 2nd account on a different instance, subscribed to nothing on both, and filtered by all>hot for both, and sure enough the content I see is way different. Some posts I can’t even find on my other account when I search for it.
This would be a great feature, but if it ever gets implemented it would have to default to show all users. All the echo chambers we saw on “the old site” would probably be amplified here otherwise.