I thought it I validates the attestation chain so the TPM won't play ball. Hang on...

https://superuser.com/questions/1278841/bitlocker-asking-for...

I cant find the reference I'm looking for which was a decent explanation about why the days of traditional style dual booting where the Linux boot loader let's you choose between Linux or Windows using GRUB to choose between Linux and Windows were over, but that question basically covers the same ground.

On dual-boot machines, I've always prefered to use the built-in UEFI boot manager (yes, UEFI has one!). This way, Linux boots using systemd-boot/grub/whaever, Windows using NTLDR and whatever else is there, it uses its native way too. It avoids exactly this, where the TPM doesn't consider that there might be an additional component, properly signed, but might cause havoc with sealed values.

Indeed, I only meant that dual booting with GRUB (other boot loaders are available) in the boot chain are over. At least, if you want to use bootlocker with keys in the TPM anyway!