@jackluke it's normal this behaviour creating install media? Thank you!
sudo /Applications/Install\ macOS\ Big\ Sur\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Install\ macOS
objc[2406]: Class IASNightInstallSupport is implemented in both /Applications/Install macOS Big Sur Beta.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/IAESD.framework/Versions/A/Frameworks/IAInstallerUtilities.framework/Versions/A/IAInstallerUtilities (0x1152abac0) and /System/Library/PrivateFrameworks/IASUtilities.framework/Versions/A/IASUtilities (0x102ab4080). One of the two will be used. Which one is undefined.
objc[2406]: Class IASSystemInformation is implemented in both /Applications/Install macOS Big Sur Beta.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/IAESD.framework/Versions/A/Frameworks/IAInstallerUtilities.framework/Versions/A/IAInstallerUtilities (0x1152abbd8) and /System/Library/PrivateFrameworks/IASUtilities.framework/Versions/A/IASUtilities (0x102ab41c0). One of the two will be used. Which one is undefined.
Ready to start.
To continue we need to erase the volume at /Volumes/Install macOS.
If you wish to continue type (Y) then press return: