No, it's not a federation issue. What we need is some syntax to link to posts/comments in an instance agnostic way. When you click something like !chat@beehaw.org your UI will take you to your instance's version of it. Clicking a URL will take you to that URL always. As expected, post A in community X hosted on instance blah will have a different URL than post A in community X viewed from instance foo. You can share a URL to the actual post on its actual home instance or you could share a URL to your instance's view of the post. That second one is useful to you and members of your instance but not members from any other instance.

Short of your UI intercepting and redirecting URLs (a very bad thing to do in my opinion, links should go where they say they go without interference) this cannot be solved with URLs. That's why we need some way to specify something like ! community@instance/post.

A big caveat is that I mean HTTP URLs. If there was a custom URL scheme like lemmy: then the rules can be different.