I noticed that a lot of the communities that are hosted on there only have posts from a week ago. Thought they were just a bit dead, but checked on their original instance and saw they're more lively than that it's just not showing up on Kbin.
As far as I know, lemmy.ml itself is not blocking kbin requests, at least not on purpose.
But I will try to get some official information from one of the lemmy.ml admins.
There were some issues with the federation of non-Lemmy instances in general.
Multiple issues caused these and affected incoming and outgoing communication.
The latest Lemmy version 0.18.1, which was released yesterday, should fix most of the issues, but some instances still need some fixing on the nginx side.