When logging in with a Mojang account an invalid "--xuid" run argument without a value is passed to the game.  This works as expected when using a MS account.

"--clientId MW<redacted>Fi --xuid  -userType mojang --versionType release"

Note the double space between --xuid and --userType, the expected cli arguments would be:

"--clientId MW<redacted>Fi --userType mojang --versionType release"

I dont belive this causes an issue in vanilla (I havent looked into it too much), however it does cause the Fabric mod loader to stumble over parsing the arguments correctly, leading to a lack of Fabric branding on the main menu.

It would be great to have this returned to the previous behaviour (without the empty --xuid argument) to ensure that all versions of Fabric work as expected for Mojang account holders. I have created a PR for Fabric Loader here with a work around: https://github.com/FabricMC/fabric-loader/pull/566

Thanks for your time, let me know if you need any more information.