> A normal reboot always [forces the TPM pin entry screen], even a 'hot' reboot.

In TPM-only mode, I only see the screen—which asks for an recovery key that serves an alternative to the TPM-borne secret, not for whatever you are calling the “TPM PIN” here—whenever I update the firmware or the bootloader (the latter from the other side of the dual-boot setup). Otherwise it boots straight to the login screen, which meshes with the measured-boot-only theory of operation I’ve described above. There’s nothing nefarious in this part, even if I think it exposes an unwisely large attack surface (e.g. the USB stack). I suspect you simply reboot so rarely you’re never hitting the happy path.