This is by design. ActivityPub is a push model and it's a Push-once (in general).
What that means is that the instance that the creator/author of the content is on will push the content to all the other instances that it knows are subscribers to the "account/magazine" that the content is being pushed to.
What that means is:
Feddit.uk will push the content to Feddia.io because that's the "destination" account/magazine.
Feddit.uk will push the content to any instances that is also happens to know subscribe to the magazine "ukdtt"
Fedia.io will not re-push the content to all the instances that subscribe. That's the responsibility of the instance that the content "originated" from, not Fedia.io
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