I see the same thing on my M1 MacBook Pro, but the system does report that FileVault is enabled.
Here's another weird thing: I usually set my laptops to not show a list of users at the logon screen. Instead, I need to type the user name and password in. On all of my other Macs (all Intel) running Big Sur (any version), when I boot the Mac, I'm presented with a list of users at the logon screen, no matter what my login options dictate. Once the disk is unlocked, the logon screen reverts to a the two text boxes for username and password, and the list of users is gone. This was the normal FileVault behavior on Intel Macs forever. On my M1 Mac with FileVault enabled, when I boot the computer, there is no longer a list of users to select from. Instead, the system preference for the name and password field persists. Are you seeing this as well?
I'm assuming this is tied in some way to the integration between the T2 and the new Apple Silicon firmware, with UEFI out of the picture.