I think this is why you don't store the encryption key of the disk directly in the TPM but a "key to unlock the key" - that way you can enter a recovery code or something to access if the TPM or something in the boot path fails. I don't know how the encryption mechanics work in detail but it has to work like that somehow for Bitlocker recovery to function. I know under Linux LUKS you can have up to 8 keys and each will allow access to the disk.