Player 2 is losing all inventory and data in split-screen on our new Xbox Series X because the game fails to authenticate their profile, resulting in a blank MCID and identical Device ID to the host, despite the save data working perfectly in solo play.

After migrating our primary survival world from an Xbox Series S to a new Xbox Series X via Cloud Sync, Player 2 consistently fails to load their save data and inventory when joining a local split-screen session hosted by Player 1. While Player 2 loads their correct profile and data perfectly when launching the game in Solo Mode (confirming the save file is intact), joining in split-screen forces them into a "Guest" state with a default skin, empty inventory, and an inability to gain XP. Critical troubleshooting within the Profile Settings menu reveals that during the split-screen error, Player 2 is assigned a Device ID (DID) identical to Player 1 and a completely BLANK Minecraft ID (MCID), indicating the game client is failing to authenticate Player 2's unique UUID during the multiplayer handshake. We have already performed all standard troubleshooting, including full console power cycles, clearing the Alternate MAC Address, verifying "Home Xbox" and "Multiplayer" settings, and re-adding the user profile, but the authentication failure persists on both local saves and Realms.