Board Thread:Roblox help and discussion/@comment-30754193-20191030025227/@comment-35998649-20191030032644

Supposedly it's an automated username on mobile devices or Xbox devices. It might be for new <13 accounts though. It's similar to the "P[numbers]" from Club Penguin, where the username has to be manually approved first before you can actually have your desired username.

The numbers BTW after "Robloxian" is the userID.