When launching the game, the username is set to "skinStandardCust" and wont change back. This can also happen briefly on starting of the game but then changes to your username. This happens when you are signed in so I do not think it is to do with the login. You also stay the same username when on worlds, but I have not managed to replicate it on servers or realms.

This attachment is from my computer on my account "ProliverPlays".

Hardware:

AMD Ryzen 7 2700x at 8 cores (64 Bit)

32GB RAM

1TB SSD