Hi Claus
I have confirmed with dev and have clarification for you on this.
In the windows 7 templates we use the command
in order to enable the bitlocker encryption
When the Windows 8 templates were created in 12.8 this was carried over but as can be seen in this MS TechNet article Manage-bde: tpm this does not work on windows 8 or above.
In order to enable bitlocker support in Windows 8 and above requires a design change to the templates. When we released 12.9 we removed the bitlocker parameter from the effected images to avoid any confusion.
It is already on the development backlog to add bitlocker support to these OS's but as of now there is no projection as to when this will be delivered.
Please do raise this as an idea so the community can provide feedback to product management on the need for this requirement