@user-unknown Thanks for posting in our Q&A.
FileVault encrypts your entire disk, including the operating system. This issue here may be that these credentials are not being passed to the OS, so the system will first ask for your username and password to unlock the disk, and then do so again to log into the system.
Given this situation, it is needed to check some logs based on this device to find why the credential is not passed to the OS. With Q&A limitation, it is suggested to create an online support ticket to get more help. Here is the support link;
https://learn.microsoft.com/en-us/mem/get-support
Thanks for your understanding.
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