What's wrong with federation between Kbin and rebased/pleroma/akkoma?

Does anyone happen to know why federation between Kbin and Mastodon works, but federation between Kbin and rebased, Pleroma, and Akkoma doesn't work? I've been doing a variety of tests and noticed that federation just doesn't work to those other platforms, which is strange because rebased and Mastodon aren't all that different, as far as I can see.