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.

Never heard of this rule but sounds like a cultural difference. I wish people would take their shoes off in my home but it’s not common around here.