I think that might be because there hasn’t been any activity in that community for a few months, possibly from before your instance federated with Kbin. I don’t think posts beyond a certain age will sync over when federation between instances starts.
On Kbin, there’s !cyberpunkred.
!cyberpunkred@kbin.social
I can see stuff over there, but when I try to access it through my instance it shows up as an empty community.
I think that might be because there hasn’t been any activity in that community for a few months, possibly from before your instance federated with Kbin. I don’t think posts beyond a certain age will sync over when federation between instances starts.