I created a Windows 11 25H2 Windows-to-Go stick, and for some reason it started encrypting itself after the initial setup. I realize that this shouldn't happen, but it did. The fact that I disabled the "Prevent Windows To Go from accessing internal disks" (intentionally, as I was running it on a computer that only has Linux on its internal SSD) may have something to do with it.
This obviously isn't great, as it caused unnecessary wear to my USB drive. Also pairing a USB installation with a TPM of a specific device doesn't sound appropriate for many use cases (although of course that won't truly happen automatically if a local account is used).
I believe that this could be mitigated by attaching the \Windows\System32\config\SYSTEM registry hive of the applied image after the drive is otherwise ready, and setting ControlSet001\Control\BitLocker\PreventDeviceEncryption DWORD value in that hive to 1.