It's supposed to be seamless, but there can be issues in federation between Lemmy and kbin. I log into Lemmy and have a number of subscriptions to kbin magazines. I've found it to be less than perfectly reliable. Consider they're two different platforms with two different teams. Development is running at a quick pace making it more likely for something to break. If that happens they're not always directly aware of issues between them.

Also instances don't synchronize all content between all instances. They do it on demand so if you're the first one on that particular instance to subscribe to a particular remote community then the instance will start federating content at that point.