I think this only applies to web clients, but I don't think ActivityPub's C2S protocol was ever designed to be used by arbitrary web clients. I suppose you could work around it by setting up some kind of proxy for requesting Mastodon URLs; you may need to do that anyway if the other server has authorised fetch enabled.

There are a whole bunch of technical implementation details that I disagree with when it comes to Mastodon. I wish the many MissKey forks would stabilise, because it seems a lot saner (yet every time I consider setting up a server, there has been a new fork thst everyone is switching to).