There appears to be a bug with this, as seen here:
Making a hyperlink like [link](niu.moe/@QwertySpace) will result in this happening [link](niu.moe/<a target='_blank' href='/user/QwertySpace'><a href="https://anilist.co/user/QwertySpace" target="_blank" rel="noopener noreferrer">@QwertySpace</a></a>).
In fact, in the first example, I had to put a zero-width space after the @ just to send it without it breaking. This also occurs when using html tags to create the hyperlink. The only way around it is to replace @ with @
This is due to the parsing rules of Anilist.
Anilist-specific markdown takes priority over all other markdown, which means that if it contains an @ (followed by non-whitespace), it is a username mention no matter what. This effectively means that other entities are not allowed to contain the @ character.
Since you put a usermention instead of a URL, you have not written a well-formed link.
You can use an @ in your posts, but the way its written isn't really right. Right now you can only use it if its placed in a word boundary(numbers, letters and an underscore) or it being followed by something thats not a letter or number.
I'm not very good with making MD flavours, but if the dev's don't care about any mess then something like this at least should do for the time being.
/(\s|^)@([A-Za-z0-9]+)/g (I only roughly wrote this, I'm sure there are better ways to do this)