Mastodon caches things heavily (well at least niu does).
I moved the actor entry for @firstname.lastname@example.org (which currently hosts a WIP ActivityHub implementation) from /actor/shuga to /shuga and it hasn't corrected itself.
By the way, thanks @Gargron for the helpful ActivityPub tutorial. It's a huge help!
Tutorial link: https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/
Welcome to your niu world ! We are a cute and loving international community Ｏ(≧▽≦)Ｏ !