This is something I haven't seen before. I'm curious if this may have been an issue with the API considering your account was so new.
In case this is not an issue with the api, would you mind opening the profile tab, and then going to Settings -> About -> Arctic Logs. There should be an entry that shows the raw api request including the username sent with the request. It's possible that the username is malformed in some way which would cause this issue.
The log entry I am looking for should look something like this:
[2025-03-02 01:13:49 +0000] [info] [lemmy] [performRequestWithCheckedResponse(_:)] LemmyAPI request: https://lemmy.world/api/v3/user?auth=_private_&limit=20&page=1&saved_only=false&sort=New&username=CreatureSurvive%40lemmy.world