Hello Ioticus.
Although I have been playing AACW for almost 2 years now, I recently stumbled upon the same doubt as yours
below, a quote from Pocus, regarding my doubts (relating to Huger's stack, in Norfolk) - thread about patch 1.12 beta
"I checked and this is normal, it has always been like that. At first, only stack level special orders existed, so preventing their use if the stack was fixed was logical. Then we introduced unit-level special order, but they will still be prevented in the stack is considered fixed. You have to put Huger out, do your manipulation and then put it back."
So, you have to remove either the leader from the stack or the fixed units from the stak, in order to be able to form a division.
As a further info, and IIRC,

apy:remember that any stack you create that contains even one fixed unit will also be fixed.
regards