Looking at this from Voyager you have no custom name displayed, it is just “AFKBRBChocolate@lemmy.world“. My guess is that voyager doesn't show those, same as with profile pictures.
Please don't apologize for wanting to be helpful! At least I learned about a new feature that is essentially hidden from me as a mobile only user. Good to know!
I don't have an account on lemm.ee, so that is why it likely does not come up. Not sure how to expand search within servers federated with your instance in voyager.
You have to put the instance name if you want to search for a user. I just did the same search on voyager for dubyakay@lemmy.ca and it went straight to their profile.
Otherwise yeah, the search probably defaults to only users on your server.
@Ilikecheese@lemm.ee has the right of it. @Mastengwe@lemm.ee, to search for a user not on your instance, you have search for 'username@instancedomain'. If you only search the username, it will search only in your local instance.
Edit : but when trying to tag somebody, it is able to search through other instances so maybe there is a way to implement the same technique to the search section.
Edit 2 : all my points above are for specifically how voyager behaves not how lemmyui and other clients do.
I doubt it is the nickname feature since according to the person who first brought up the feature responded to the first person bringing it up their chosen nickname is different from what I can see, which is their actual account name. Refer to the according comments further up