Mastodon is just one of many applications that uses AP for their own custom purposes. MissKey and derived software has some kind of emoji response feature to posts that’s basically unimplemented anywhere else. Lemmy’s boosting trick to make comment sync make interoperability with timeline based social media a spamfest.

Maybe I should check again, but last time I looked into it there were no commonly used ActivityPub compliant servers. Everyone does their own thing just a little different to make the protocol work for their purposes. Even similar tools (see: MissKey/Mastodon, Lemmy/Kbin) took a while to actually interoperate.

As far as I can tell, the idea behind the original design, where servers are mostly content agnostic and clients decide on rendering content in specific ways, hasn’t been executed by anyone; servers and clients have been mixed together for practical reasons and that’s why we get these issues.