Thank you everyone for your replies.
I don't understand English very well and am using a translator, so I'm sorry if I can't understand it well.
Referenced MCPE-173209
The unknown ID that is displayed is a combination of English words and numbers, not the default character name, and is only displayed while the lobby screen sign-in is loading. This may be the name set on a custom skin purchased from the Marketplace.
In that case, is it by design and not a bug?
The ID displayed is "Horns 5".
When I checked, it seems to be the name set for the skin I am using.