Why is /kbin federation with Lemmy and Mastodon usually broken?

I don't know if this is part of what OP is referring to, but the unofficial FAQ says:

Finally from Mastodon, you can Reply, Like, and Boost /kbin posts. You can also follow /kbin users and subscribe to /kbin magazines.

I admit that Mastodon users can indeed subscribe to /kbin magazines, but it seems useless to do so: even if I subscribe to a magazine from Mastodon, the content added to the magazine will not appear in its feed.