After some debugging over the weekend and into this morning we finally have federation working as it should!

Now if you search for any community or user @mtgzone.com from another federated instance (Lemmy, Mastodon, or any app in the fediverse) you'll be able to correctly follow it there.

For instance, if you have a Mastodon account at mastodon.social, you can login there and search for either https://mtgzone.com/c/mtg or @mtg@mtgzone.com and click to follow this community from your Mastodon timeline.

For the curious, ChappIO and @megan@sunny.garden in this GitHub issue finally solved this one.