Hi. There is no native endpoint of resolving user data by username so we have to combine basic users search endpoints with its alternatives. Some users could be flagged by censor and other filters, and would be only available by direct user by id request. If you have user id, it’s always better to use this
Otherwise just consider this kind of errors as worth for retry, may be with a little delay

if u want to investigate specific users, contact us with usernames via pm