why do I get the feeling I'm about to learn something?
It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.
And there it is. I'l follow up on this with our base Engineer but it would explain everything quite nicely if correct, especially Latetonites disappearing flight directors.